Grenzo was chosen due to his immense synergy with Doomsday piles.

He acts as your catalyst to crack into your pile.
He places creatures directly onto the board, preventing interaction from counterspells.
His ability is stacked at instant speed.
His ability allows us to bypass cards with Rest in Peace and Leyline of the Void replacement effects. 

Due to the nature of the replacement effect, the card will be placed into exile, and drawn back out due to Grenzo's ability being indifferent to the final zone the card ends up in, caring only about the initial move from the library to a public zone.

The desire to play Grenzo Doomsday will depend a lot on what you’re looking for in an EDH deck. If you’re looking for a deck with endless backup plans, this might not be for you. If you’re looking for a deck to impress your friends with your endless ability to chain spells, this might not be for you. If you want a deck that slows the game down to a crawl, or reanimates everything, wins with commander damage, or has game plans so long and complicated you’re positive your friends will never catch on, this deck might not be for you.

This deck is for people who;
Want a fast, efficient deck that consistently threatens a win between Turns 2-4
Want a deck with a straightforward wincon, but multiple angles to complete it
Want a deck with a straightforward backup wincon
Want a deck that utilizes your commander
Want a deck that can often get around instant speed interaction and most graveyard hate
Want a deck that plays out reliably and consistently
Want a deck that can play competitively at a price point cheaper than most semi-budget decks

The goal is to race your opponents to 5 mana, cast Grenzo, and use the wide tutor suite to find and cast Doomsday as quickly as possible. This occurs as early as turn 1, but is commonly played out on turn 3, (or turn 4 through hate.)

If, for some reason you fail to succeed in casting Doomsday, your backup is to tutor or wheel into a Recoup effect (covered in a later section), or hardcast the Redcap Combo if Doomsday becomes exiled or grasped.

Our ideal hand would include the following pieces:

2-3 Land (Multicolored or Swamp)
1-2 Artifact Ramp
1-2 Tutors (Instant speed is preferred, such as Vampiric Tutor )
1 Protection Spell (Counterspell prefered, such as Pyroblast )
1 Ritual Effect ( Dark Ritual or Rite of Flame preferred)

The primary benefit of playing Grenzo Doomsday is its speed.
Artifact ramp assures we get to cast our required spells as fast and efficiently as possible.

Color fixing mana rocks
These ensure we have the proper colors to cast the spells regardless of our drawn land, while at the same time, being played for low costs so that we can play them early and cast more spells faster.

Chrome Mox
Mox Opal
Fellwar Stone
Lotus Petal
Mox Diamond
Springleaf Drum
Rakdos Signet
Talisman of Indulgence
Jeweled Amulet
Chromatic Sphere
Chromatic Star

Colourless Ramp
The follow mana artifacts produce maximum colorless mana for minimal cost.
Remembering that a win only requires BBB+2, drawing into one of the follow mana rocks
can promote the chance to combo out a full turn earlier than without them.

Mana Crypt
Mana Vault
Sol Ring

Doomsday Ramp
A single mana artifact, but a great one, LED allows us to play Doomsday directly on BBB, rather than acquiring the following 2 mana. While it forces a hand discard, the cards in your hand are not as important as the speed to which you can play through a Doomsday pile, and LED can able that as early as T1 or T2 depending on your board state.

Lion's Eye Diamond

======================
Spell Ramp provides quick access to coloured mana, subsequently increasing the speed in which our deck can find and cast Grenzo and Doomsday.

Ritual Effects
Rite of Flame
Cabal Ritual
Dark Ritual
Desperate Ritual
Pyretic Ritual
Culling the Weak

Land Ramp
Bubbling Muck
Rain of Filth

Creature Ramp
Blood Pet - A great first turn ramp drop if there are no other 1 drops playable in hand.
Simian Spirit Guide - Often useful as a way to go back to back swamps in your opening hand, so that Grenzo can be cast on curve.
Overeager Apprentice - BBB is life, and Apprentice provides.

Kiki-Jiki Pile #1 - Zealous Conscripts:
The goal is to rush to cast Grenzo, cast Doomsday, and win. We do this by
casting Doomsday when Grenzo is in play, and you have BBB+2 mana available.

-With Grenzo in play, cast Doomsday for BBB, leaving you with 2 mana,

-Pile the following, from Top to Bottom: Dark Ritual, Zealous Conscripts, Goblin Sledder, Kiki-Jiki, Priest of Gix,

-Spend 2 mana, activate Grenzo, to bring in Priest of Gix , and acquiring BBB,

-Spend 2 mana, activate Grenzo, to bring in Kiki-Jiki, who we 
  then activate making a copy of Priest of Gix, pooling BBBB,

-Spend 2 mana, activate Grenzo, to bring in Goblin Sledder, 
 who will sacrifice himself to add +1/+1 to Grenzo, leaving you with BB pooled,

-Spend 2 mana, activating Grenzo, to bring in Zealous Conscripts , who will take control of Kiki-Jiki and untap him,

-Kiki-Jiki will then go infinite with Zealous, creating infinite tokens with haste, who then attack and win.

-Dark Ritual is left on the pile as a way to recast Grenzo if he somehow dies during the Doomsday Turn. 
 It will be naturally draw on the next upkeep (if there is one), and the mana can help pay the tax and ability cost.

===
Kiki-Jiki Pile #2 - No Zealous Conscripts:
The goal is to rush to cast Grenzo, cast Doomsday, and win. We do this by
casting Doomsday when Grenzo is in play, and you have BBB+2 mana available.

-With Grenzo in play, cast Doomsday for BBB, leaving you with 2 mana,

-Pile, from Top to Bottom: Metallic Mimic, Murderous Redcap, Skirk Prospector, Kiki-Jiki, Priest of Gix,

-Spend 2 mana, activating Grenzo, to bring in Priest of Gix , leaving you with BBB,

-Spend 2 mana, activating Grenzo, to bring in Kiki-Jiki, Mirror Breaker, 
 who will then activate making a copy of Priest of Gix, pooling BBBB,

-Spend 2 mana, activating Grenzo, to bring in Skirk Prospector , leaving you with BB,

-Spend 2 mana, activating Grenzo, to bring in Murderous Redcap,

-Sacrifice Kiki-Jiki and Redcap to Skirk Prostpector, giving you RR,

-With Redcap’s Persist trigger on the stack, activate Grenzo again with 
 your remaining mana to bring in Metallic Mimic naming Goblin,

-This returns Murderous Redcap without any counters, allowing you to go infinite with Redcap 
  being sacrificed to Skirk, while returning with no counters due to Mimic. This creates an infinite 
  loop of 2 damage dealt, killing the table.

===
No Kiki-Jiki in Pile:
If Kiki-Jiki has been removed from the game, our pile changes.
This pile will rush to cast Grenzo, cast Doomsday, and win. We do these by
casting Doomsday when Grenzo is in play, and you have BBB+3 mana available.

-With Grenzo in play, cast Doomsday for BBB, leaving you with 3 mana,

-Pile, from Top to Bottom: Metallic Mimic , Murderous Redcap , Skirk Prospector , Priest of Urabrask , Priest of Gix,

-Spend 2 mana, activate Grenzo, to bring in Priest of Gix , leaving you with BBB+1,

-Spend 2 mana, activate Grenzo, to bring in Priest of Urabrask leaving you with BBRRR,

-Spend 2 mana, activate Grenzo, to bring in Skirk Prospector leaving you with BBR,

-Spend 2 mana, activate Grenzo, to bring in Murderous Redcap leaving you with B,

-Sacrifice Redcap to Skirk Prospector, adding R, leaving you with BR,

-With Redcap’s Persist trigger on the stack, activate Grenzo again with your 
  remaining mana to bring in Metallic Mimic naming Goblin,

-This returns Murderous Redcap without any counters, allowing you to go infinite with 
 Redcap being sacrificed to Skirk, while returning with no counters due to Mimic. 
 This creates an infinite loop of 2 damage dealt, killing the table.

===
No Doomsday:
If Doomday is countered, your goal is to assemble the Murderous Redcap , Metallic Mimic ,
and one of our sacrifice outlets ( Skirk Prospector , Goblin Sledder , Mogg Raider ).

-With those pieces in play, and Mimic naming “goblin””, Redcap goes infinite damage with a sacrifice outlet.

Grenzo runs as many usable tutors as possible in order to find Doomsday as quickly and efficiently as it can be in the early turns.

Vampiric Tutor - Ideally in your opening hand, Doomsday goes neatly to the top of your library.
Imperial Seal - The same as Vampiric Tutor.
Demonic Tutor - Puts the desired card directly in hand.
Diabolic Intent - Doomsday allows us to recur most of our creatures, so we can use them as pitch to Intent.
Dimir Machinations - This card is cast for it’s transmute cost, allowing us to find 
                most of its main cards like Doomsday , Yawgmoth's Will , Toxic Deluge , etc
Gamble - This is your "hail mary" tutor! If you really want to go fast, this one will do it for you, 
                but beware of the chance to discard your own wincon.
Grim Tutor - A little costly, but added for redundancy.
Plunge into Darkness - Not really a tutor, but we're going to use it as close to one as we can 
                when all we need is Doomsday or a piece of ramp. Because this is risky in that it 
                can exile combo pieces while hunting for an enabler, I have found that I often will 
                stop my plunge at 8-10 cards, rarely going more than that, or you risk losing a 
                piece from every combo.

======================
Rakdos colors make our land selections very obvious, as we need as much black and red as possible, as fast as possible. For this, we run 30 lands, 1 more than the classic 29 land base that’s become popular, simply because we often want to start with 3 lands in hand, or have enough to ensure we get more on wheels.

Fetch Lands:
We run all 7 dual color suitable fetch lands. It is important for this deck to open with as much color variety as possible, allowing us access to both black and red to cast Grenzo, and enough black mana for Doomsday.

Fetch Targets:
These are the lands you’ll want to target with your fetches. Doing so ensures we have adequate coverage of our color need to both cast Grenzo, as well as Doomsday the following turn.

Badlands
Blood Crypt

Going Fast Lands
These lands are used due to their synergy with our Turn 3 win plan. While many decks may find them sub-optimal, Grenzo requires a direct effort to maximize the speed of our available mana in the first three turns, because for Grenzo, there is not much of a late game.

Peat Bog

Utility Lands:

Cavern of Souls - Naming “Goblin” allows us to jam Grenzo and most of the parts of 
         our alternate wincons without having to worry about immediate counterspell interaction.
Lake of the Dead - The ability of this land allows us to play out Doomsday and the subsequent 
        piles if we don’t happen to draw suitable artifact ramp. Tapping a Swamp for B, then playing 
        Lake and sacrificing that Swamp allows us to aquire BBBBB with simply two lands. 
        Remembering 5 mana is all we need to Doomsday and win, Lake allows us to do that 
        while fighting through land destruction and artifact hate.
Urborg, Tomb of Yawgmoth - Urborg allows us to ensure we hit our mandatory 3 swamps for Doomsday, 
        and works to color fix against possible land hate. It remains weak to Blood Moon effects, but can 
        and will save you if someone happens to remove your black producing lands.

The Rest of the Lands
The remainder of our lands are included to provide fast access to colors, untapped, on the turn we play the land. We run 10 basic Swamp and 1 Mountain to play around non-basic land destruction. 10 swamps gives us our best swamp-to-muck/filth ratio without cutting into our red production. Ideally 11 swamps would be included, but we've found that ideal muck lines were affecting our red consistency too greatly.

Sometimes, you just don’t want other players cards on the board, so we run enough removal to take care of those issues.

Pyroclasm
Shattering Spree
Toxic Deluge
Smelt
Chaos Warp
Dismember
Rakdos Charm
Slaughter Pact
Fire Covenant

======================
Proactive and Reactive disruption are important to have as being affected by removal/hate after Doomsday has been cast becomes particularly important.

Proactive Disruption
Being able to look at and remove pesky interaction cards from our opponents hands prove a cheap way to get around counterspells and disruption.

Duress
Inquisition of Kozilek
Thoughtseize
Praetor's Grasp

Slot in the following to ensure opponents struggle to interact with us as we Doomsday off;

Defense Grid
Overmaster

Reactive Disruption
Sometimes, the best way to prevent losing, is just a good old counterspell.

Burnout
Pyroblast
Red Elemental Blast

Graveyard Disruption
Graveyard disruption isn't something that needs to be heavily invested in, but you'll be happy you have it.

Rakdos Charm

========================
Sadly, Grenzo doesn't offer you a ton of flex position with how all-in on the race the deck is! That said, there are certainly spots that allow some leeway specifically depending on what you need out of the deck!

The cards below are often in flux, and are continuously removed and re-added as we hunt down the perfection of Grenzo.

Burnout , Pyroblast , Red Elemental Blast

These are the cards that certainly shouldnt remain in the deck if your meta happens to be Blue-Lite!

Toxic Deluge , Pyroclasm . Fire Convenant

These can certainly be replaced/brought in based on your meta with alternative forms of removal.

Defense Grid , Conqueror's Flail

A nice meta choice to play on your Doomsday turn to protect you from counterspells and removal, but certainly not necessary.

Night's Whisper , Dark Confidant

If your meta is all-in race combo's there's no reason to include passive card draw. In these pods, you're going to win big or lose big, so the game won't continue long enough to need these.

Undying Evil

Fantastic if your meta is heavy on instant speed destroy effects, but unhelpful if that removal is instant speed exile.

Like every deck, there are often cards and combos that do not appear in this list that are often used in others. Sometimes there are cards that exist that seem like a perfect fit, but then don’t end up appearing. Here are some of the notable exclusions for Grenzo Doomsday

Worldgorger Dragon + Animate Dead

This combo is often found in many Rakdos shells, especially in Doomsday. It promotes a system of infinite mana, and then infinite activations of Grenzo to draw the entire deck out until a combo or win condition is found.

It has been excluded from this deck with the discovered of the Metallic Mimic combo listed in the combo section above.

The reasons for exclusion are thusly;

The cards needed for assembly of Dragon combo are many and varied, 4-5 card for reanimation and setup, as well as the Dragon itself lead to having between 8-9 cards in the deck dedicated only to creating this combo, but adding no value to any other aspect of our primary gameplan

The Mimic combo only requires 6 cards, those cards can be used individually as a natural cast combo, added to a Doomsday pile, and operate as a backup pile when Kiki-Jiki is removed from the game with little to no additional setup time.

An ill-fated counterspell on Dragon Combo will end the game as quickly as it starts, and with the availability of disruption in EDH, it was more likely to happen than not.

Yahenni's Expertise

This card is often toted as being a suitable replacement for Toxic Deluge. At face value, it essentially reads “Pay 1 extra mana, to add a -3/-3 boardwipe to your 3 cost spell”. It’s exclusion is for the following reasons;

It costs an additional mana over Toxic Deluge, and 2 additional over our other board wipes.

It requires that you’re holding a 3 mana spell of value. If you are not, then you’re either jamming a spell simply to play it, or you’re not casting a spell, and instead playing 4 mana just to add a -3/-3

Yaheeni’s cannot be altered lower or higher based on need. Deluge can be cast for 1 to sweep stax peices without harming Grenzo, and it can be cast for 4+ to hit other hate such as Loxodon Gatekeeper , or other high toughness threats like a cheated in Elesh Norn. Yaheeni does not provide that utility.

Seething Song

Seething Song is simply removed due to its high CMC cost and it's massive overproduction of Red mana, which is not heavily needed in this deck.