Slivers (EDH / Commander)

Introduction

For a deck that's supposed to practically build itself, there's still an enormous number of card choices to make. Perhaps that's a problem with 5-color commanders in general...

This build revolves around activated abilities. With Sliver Overlord in the command zone & many other slivers with activated abilities, our main goal is to land a Training Grounds. From there we accrue massive value, for example:
-- Sliver Overlord: 1-mana demonic tutors
-- Necrotic Sliver: 1-mana instant-speed Vindicates
-- Spectral Sliver: Turn all our slivers into shades
-- Sliver Queen: Infinite combos

With Unnatural Selection in play, Sliver Overlord gives us an 2-mana, instant-speed Control Magic. Repeatedly stealing the biggest threats at the table will probably generate enough value to control the game. But it also enables many weird shenanigans:
-- Turn an opponent's attacker or defender into a Sliver to decide a combat
-- Melt auras with Ward Sliver & 'gifting' color protection
-- Blank removal (or equip!) targeting an opponent's creature
-- Temporarily reduce synergy in opposing tribal/typal decks

You do have to be careful turning opponents' creatures into slivers, as you'll likely give a number of other potentially unwanted abilities in the process. It would be a sad day to hand someone a Vindicate that they then use on one of your permanents...

Strategy

As a creature type, Slivers are one of the most toolbox-y. Overlord support is especially helpful here. There's a Sliver-based solution for most board states:
-- Big creatures: Necrotic Sliver // Striking Sliver + Venom Sliver
-- Lots of creatures: Shifting Sliver
-- Pesky creatures: Hunter Sliver
-- Annoying artifacts & enchantments: Harmonic Sliver
-- You're playing archenemy: Sentinel Sliver

Never under-estimate Quick Sliver.

The most effective wincon is undoubtedly Sliver Queen. There are many ways to trigger an infinite combo that wins on the spot:
-- Piece 1: Training Grounds // Biomancer's Familiar // Heartstone
-- Piece 2: Basal Sliver // Ashnod's Altar
-- Piece 3: Sliver Overlord // Acidic Sliver // Heart Sliver

Though, personally I'd prefer to win in "fairly" with a massive army pumped by Sliver Legion or Coat of Arms. :-)

This deck obviously needs creatures on board. As such, there's a solid suite of countermagic & tricks aimed to protect Slivers from Wrath effects. (Crystalline Sliver & Steely Resolve are great protection against targeted removal.) If necessary, the deck can also reassemble with Dregscape Sliver, Sliver Gravemother, or Patriarch's Bidding. A single Humility also generally ruins our day.

The bigger problem is fast combo decks, especially creature-based ones. Most of the countermagic is aimed at non-creature spells, so a deck like Rocco, Cabaretti Caterer or The Beamtown Bullies blanks much of our interaction. Consider running more hate pieces (Drannith Magistrate, Gaddock Teeg, Leyline of the Void, etc.) if your playgroup runs these kinds of decks. We already have a great tutor suite to get the pieces you need.

Alternate Cards

-- Myriad doesn't make the cut since most of our creatures aren't all that great in multiples
-- Eerie Interlude / Ghostway are not quite there since we have few ETBs. Phasing and grave recursion are better for us
-- Urza's Incubator looks good on paper, but we don't need the large mana discount. Most of the deck is low CMC, and many slivers don't even have a colorless pip in their costs
-- Aether Vial is great if it's in our opening hand. Otherwise, it's a little too slow. We definitely don't want to draw it after Turn 3