Introduction

This deck was born while I was looking for Doomsday piles to put into my first Marchesa's deck.
I found this article about Doomsday with Grenzo as a commander, it was very well explained and it came with a decklist too (I'd put the link, but I think the website has been taken down time ago).
I thought it was a really cool idea and immediately started to brew my own version. It came together quite fast and easily, and in a matter of few months I already had a stable build.
It was quite different from the original, though, since that was aimed for competitive 1-vs-1, while mine was intended for multiplayer.
At some point, I had to decide if I wanted to push it for competitive play or keep it at a more casual level. I finally decided for the second option because I've never really played competitive, so I didn't feel like I was up to the task. Moreover, even if I wasn't planning to build it in the real life, many of the cards (Mana Crypt, Imperial Seal...) were far too expensive to justify the whole thing.
By the way, the deck is still quite strong. I'd say, in a rating scale from 1 to 5 (where 1 means "casual" and 5 means "competitive") that this deck scores 3 to 4.

Gameplay

The deck could win as soon as turn 3, provided you cast Grenzo, Dungeon Warden on turn 2 and follow up with Doomsday the next turn, with something like Dark Ritual or Sol Ring that lets you have 2 spare mana open.
In some situation, you could even spend turn 1 casting Vampiric Tutor to search for missing combo pieces and still be able to go off at turn 3.
The deck mostly relies on flipping creatures from the bottom of your library with your commander, so if your opponents find a way to disrupt your strategy or you simply run into bad luck while flipping cards, you're pretty much dead.
To mitigate this, the deck runs a decent amount of creatures, most of them having power 2 or less. In fact, more often than not you won't need to put any mana into the X part of Grenzo's casting cost.
Moreover, there are several tools that help you to stack the bottom of your library, and many tutors to find the cards you need.
The best moment to flip a card is generally at the end of turn of the last opponent that gets to play before you, so that your creatures don't die to sorcery-speed wrath effects. If you have Thran Turbine out, you can wait until your upkeep but flipping after your draw step is a risk, because if you happen to flip Maralen of the Mornsong all our opponents will get to tutor whatever they want before you.

> Combos

The deck runs many combos. In fact, it's not that unusual to accidentally assemble one of them by just randomly flipping cards with Grenzo.
Here's the list:

• Kiki-Jiki, Mirror Breaker + Zealous Conscripts
  This is probably the easiest combo to assemble, since it only requires 2 cards and you only need to find a way to pump Grenzo's power by 1 to flip them from the bottom of your library.
  The only disadvantage is that you have to pass through your combat phase, so you can't win at instant speed with this.
  For those who don't know the combo, you basically tap Kiki-Jiki to copy Zealous Conscripts. When the token enters the battlefield you target Kiki-Jiki, gaining control (unrelevant) and untapping it. You can now repeat the process as many times as you want, making infinite hasty tokens and attacking everyone for lethal damage.

• Kiki-Jiki, Mirror Breaker + Skirk Prospector + Lightning Crafter
  This combo requires 3 cards but can win at instant speed, since it relies on direct damage and doesn't need to pass through the combat phase.
  There are other sacrifice outlets in the deck that you can use (Viscera Seer and Ashnod's Altar), but Skirk Prospector is slightly better because it can make mana in the process and it's a goblin. This is particularly important because, as you'll get to see in a moment, this deck has several ways to fetch goblins and there are several cards that care about goblins in general.
  The combo works as follows: you copy Lightning Crafter with Kiki-Jiki, the token champions Kiki-Jiki, then you tap it to deal damage and sacrifice it. Kiki-Jiki returns to play untapped, so that you can repeat the process as many times you want until you kill everybody.

• Mikaeus, the Unhallowed + Triskelion
  This combo requires only 2 cards and can win at istant speed. It's normally the combo you go for when you have Victimize in your hand, since Mikaeus' power of 5 makes him especially tricky to flip with Grenzo (as you'll see, this deck has some way to do that nonetheless).
  How the combo works: Triskelion is a 2/2 (because it's pumped by Mikaeus) with three +1/+1 counters on it. You remove one counter to ping something for 1, then you use the rest to kill Triskelion itself. It will come back with 4 counters thanks to undying. You can now ping anything for 2 and keep killing Triskelion, so that it will return each time with new counters on it, until you get to kill everybody.

• Murderous Redcap + Viscera Seer + Metallic Mimic
  This is the most recent addition to Grenzo's arsenal. It's essentialy a variant of the previous combo (in fact, you can use Mikaeus instead of Metallic Mimic) with the upside that you don't need to pump Grenzo to flip it, since all creatures have power 2 or less.
  Again, it has the downside of consiting of 3 cards rather than 2. Moreover, you can swap Viscera Seer for Skirk Prospector or Ashnod's Altar too.
  Here, you sacrifice Murderous Redcap so that it will come back to play because of persist, but the -1/-1 counter will be elided with the +1/+1 counter granted by Metallic Mimic. You can repeat the process infinite times, shooting evreybody to death thanks to Redcap's triggered ability.

• Maralen of the Mornsong + Stranglehold
  This isn't technically a combo, since you're not immediately winning the game, but it's a great way to lockdown the board.
  Also, it could be considered a 1-card combo, since you only need to flip Maralen and she will tutor for Stranglehold by herself. Therefore, it's a good pile for Insidious Dreams.
  If you have both cards in play, on your turn you'll pay 3 life to tutor for whatever, on other players' turn they'll still pay 3 life but can't tutor for anything because of Stranglehold. Your opponents will effectively be prevented from drawing new cards, so you should eventually win the game.
  As I said before, you have to be the first player to take advantage of her ability for this to work. Additionally, make sure to have enough mana to cast Stranglehold on that same turn.

> Doomsday Piles

Here I will detail some notable piles you can stack with Doomsday.
It's important to note that they're meant to be a guideline and not an exhaustive list. In fact, sometimes you could have some combo pieces already on the battlefield (or in your hand), other times you could have more or less mana available for extra protection and so on. You should adapt them to the game situation.

• Priest of Gix - Kiki-Jiki, Mirror Breaker - Mad Auntie - Zealous Conscripts
  This is the basic pile and the cheapest one, since it only requires 4 cards and 2 mana to go off.
  Steps to follow:

  • pay 2 mana to flip Priest of Gix and net 3 black mana in exchange.
  • use 2 of those 3 mana to flip Kiki-Jiki, Mirror Breaker, tap it to clone Priest of Gix and net 3 more black mana (4 total).
  • use 2 of that mana to flip Mad Auntie and pump Grenzo, Dungeon Warden.
  • now that Grenzo is a 3/3, you can use the last 2 mana to flip Zealous Conscripts and go infinite with Kiki-Jiki.

• Treasonous Ogre - Clone Shell - Lightning Crafter - Kiki-Jiki, Mirror Breaker - Viscera Seer
  This pile consisits of 5 cards but still requires 2 mana, provided you have enough life to activate Treasonous.
  Steps to follow:

  • pay 2 mana to flip Treasonous Ogre, you can use it to get the 6 mana you'll need later on.
  • pay 2 mana to flip Clone Shell, look at the other creatures and imprint Lightning Crafter.
  • pay 2 more mana to flip Kiki-Jiki, Mirror Breaker.
  • pay the last 2 mana to flip Viscera Seer, then sacrifice Clone Shell to get Lightning Crafter back and go infinite.

• Priest of Gix - Kiki-Jiki, Mirror Breaker - Murderous Redcap - Skirk Prospector - Metallic Mimic
  This pile starts like the first one, but then uses Kiki-Jiki only to make mana. It only requires 2 mana but needs 5 cards.
  Steps to follow:

  • pay 2 mana to flip Priest of Gix and net 3 black mana in exchange.
  • use 2 of those 3 mana to flip Kiki-Jiki, Mirror Breaker, tap it to clone Priest of Gix and net 3 more black mana (4 total).
  • use 2 of that mana to flip Murderous Redcap, shoot whatever you want for 2 damage.
  • use the last 2 mana floating to flip Skirk Prospector, then sacrifice Kiki-Jiki and the Redcap to net 2 red mana.
  • in response to Redcap's persist trigger, use that mana to flip Metallic Mimic.
  • Murderous Redcap will return to the battlefield with no counters, so you can go infinite.

• Ransack the Lab - Victimize - Mikaeus, the Unhallowed - Triskelion.
  This pile doesn't need Grenzo to go off, the only requirement is to have 3BB mana open and one creature on the battlefield.
  Steps to follow:

  • draw Ransack the Lab and cast it, keeping Victimize while dropping Mikaeus, the Unhallowed and Triskelion to the graveyard.
  • cast Victimize to reanimate them and win the game.

> Insidious Dreams Piles

Many of the piles for Doomsday are essentially the same for Insidious Dreams, with an additional card to put the rest on the bottom of your library.
All piles require you to draw a card, so the better moment to cast Insidious Dreams is right before starting your turn.
Unfortunately, you won't have slots available for mana acceleration, so the focus here is more on the cards you have to discard to pay for Insidious Dreams' additional cost.

• Howltooth Hollow/Spinerock Knoll - Kiki-Jiki, Mirror Breaker - Mad Auntie - Zealous Conscripts
  This is the Insidious Dreams version of Doomsday's first pile.

  • draw the hideaway land and play it.
  • put Kiki-Jiki, Mad Auntie and Zealous Conscripts on the bottom of your library, hiding away the fourth card.
  • flip the cards with Grenzo and win the game (you need 6 mana to do so).

• Temple of Malice - Brass Herald - Kiki-Jiki, Mirror Breaker - Zealous Conscripts
  This is essentially the previous pile, with Temple of Malice instead of the hideaway land.

  • draw Temple of Malice and play it.
  • scry the top card of your library, putting Brass Herald on the bottom.
  • flip Brass Herald, naming rogue to pump Grenzo, and put the rest on the bottom of your library.
  • flip Kiki-Jiki, Mirror Breaker and Zealous Conscripts, then go infinite.

• Howltooth Hollow/Spinerock Knoll - Murderous Redcap - Skirk Prospector - Metallic Mimic
  Same as the first pile, but with Murderous Redcap combo instead.
  It costs 1 less mana, because you get to sacrifice the Redcap to Skirk Prospector during the process.

• Temple of Malice - Clone Shell - Mikaeus, the Unhallowed - Viscera Seer - Triskelion
  This pile requires one more card to go off.

  • draw Temple of Malice and play it.
  • scry the top card of your library, putting Clone Shell on the bottom.
  • flip Clone Shell, imprint Mikaeus and put the rest on the bottom.
  • flip Viscera Seer, then sacrifice Clone Shell to get Mikaeus back.
  • flip Triskelion and go infinite.

• Howltooth Hollow/Spinerock Knoll - Brass Herald - Goblin Recruiter
  This pile only requires 3 cards, although you need 8 mana to set it up.
  The trick here is to use the first pile to fetch a second one (namely, the one with only goblins) and go off from that.

  • draw the hideaway land and play it.
  • look at the top of your library, putting Brass Herald and Goblin Recruiter on the bottom.
  • flip Brass Herald, naming Rogue to pump Grenzo.
  • in response to Brass Herald's trigger, flip Goblin Recruiter and fetch Skirk Prospector, Sling-Gang Lieutenant, Kiki-Jiki, Mirror Breaker and Lightning Crafter.
  • flip Skirk Prospector (you've paid 6 mana so far), then sacrifice Goblin Recruiter to net 1 red mana.
  • pay 1 more mana (7 so far) to flip Sling-Gang Lieutenant, then sacrifice it together with its tokens to net 3 mana.
  • use 2 of that mana to flip Kiki-Jiki, Mirror Breaker.
  • pay 1 more mana (8 total) to flip Lightning Crafter and go infinite.

• Howltooth Hollow/Spinerock Knoll/Temple of Malice - Maralen of the Mornsong
  This can be actually pulled off any time you happen to flip Maralen of the Mornsong.

  • draw the land and play it.
  • put Maralen of the Mornsong on the bottom of your library.
  • at the end step before starting your turn, flip Maralen.
  • on your draw step, tutor for Stranglehold and cast it to lock the board.

> Other Interactions

• Moggcatcher
  People often overlook this card, but by resolving this critter you're basically putting a clock of 2 turns on the game.
  Of course, you have to wait for a whole round table before starting to use it (because of summoning sickness), but after that you can follow these steps:

  • at the end step before starting your turn, tap Moggcatcher to fetch Kiki-Jiki, Mirror Breaker.
  • tap Kiki-Jiki to copy Moggcatcher, the token will survive through your turn because the end step already passed.
  • during your turn, tap both the original Moggcatcher and the token to fetch Skirk Prospector and Lightning Crafter (you need 6 mana to do so).
  • win the game.

• Wort, Boggart Auntie + Gempalm Incinerator
  This falls into the casual side of this deck. In fact, Wort was one of the reasons that made me steer away from a full-competitive decklist.
  I like the card, it's pretty solid by itself. Most of the creatures in this deck have weak stats and tend to die to whatever, Wort gives you a way to recur at least part of them.
  In addition, with Wort in play, having Gempalm Incinerator in your hand means you can pay 2 mana each turn to draw a card and, provided you have enough goblins on the battlefield, to uncounterably kill some creature in the process.

• Sling-Gang Lieutenant + Ashnod's Altar/Mana Echoes + Reito Lantern
  This is actually yet another combo. I'm listing it here only because the deck doesn't really support it.
  Basically, you can swap Sling-Gang Lieutenant for any token-maker that makes 2 or more tokens and get the same result.
  The problem is, if you don't get either Ashnod's Altar or Mana Echoes into play, all these token-makers are quite unimpressing. And if I had Demonic Tutor in my hand, I'd rather go for Doomsday than waste my time going wide with tokens.
  By the way, all these cards are quite good by themselves, so they're in the deck regardless of the combo.

> Card Analysis

TODO

> Notable Exclusions

• Sensei's Divining Top
  TODO
• Worldgorger Dragon
  TODO
• Braid of Fire
  TODO
• Junktroller/Canal Dredger/Epitaph Golem
  TODO
• Soldevi Digger
  TODO
• Tel-Jilad Stylus
  TODO