Mairsil, the Pretender is a commander that can take bad cards, which in nearly any other deck would have massive drawbacks , and extract powerful engines or effects. Mairsil can also subvert expectations, typically when my opponents see Skithiryx, the Blight Dragon or Kozilek, the Great Distortion get exiled. The only card that needs to resolve all game is Mairsil, the Pretender, and I regularly have 8 cards in exile to puzzle over engines with. An important concept for this deck is that when Mairsil changes zones, it's now a new object and therefore can use each ability once more. For example, Commander's Sphere + Cauldron of Souls is some good card advantage with a way to reset counters like Carnifex Demon or Endling.
The goal of the deck is to set up a convoluted combo using only my commander, emphasis on convoluted. I know that the most efficient combos for Mairsil use Gilded Lotus, but that's a bit too straightforward so instead I use Lotus Bloom, Soldevi Adnate, Astral Cornucopia, and/or Empowered Autogenerator. This is obviously slower, so the deck needs to have a decent control game, which is accomplished with Nevinyrral's Disk, Perilous Vault, and Oblivion Stone as repeated boardwipes from Mairsil as well as repeatable targeted removal. Keeping Mairsil in play can also be difficult, so there are a large number of protection abilities to regenerate, gain shroud, become land, flicker, undie, or just make an new Mairsil.
Commander
Spectators outside the cage
Spectators that enter the cage
Ramp up to (and into) the cage
Untapping effects
Never dying, not for long
Bringing a grenade to a cage fight
Charging the laser
Hidden capacitor bank
Building the orbital railgun
Surviving to monologue
Some notable single cards:
- Pendant of Prosperity see you as the owner and the controller, so it's : draw 2 and play 2 lands.
- Thassa, Deep-Dwelling cages 1 card/turn, and can prevent attacks in a pinch. Cage-able.
- Aetherling to cage additional cards and avoid removal.
- Dralnu, Lich Lord to flashback the tutor
- A haste enabler, either Anger if I have a mountain or Skithiryx, the Blight Dragon if I don't.
- Endling if I already have a sacrifice outlet in hand/land because endling has more combo potential.
- Dralnu, Lich Lord to flashback the tutor
- Nevinyrral's Disk if the opponents are setting up quickly (via Intuition) or Cinderhaze Wretch/Leech Bonder to reset the counter for Endling.
- Aetherling + Arcanis the Omnipotent + haste
- Commander's Sphere + Endling + Cinderhaze Wretch + haste
- Tree of Perdition + Hateflayer. Mairsil is 4/4, so this sets them to 4 life then deals 4 damage.
- Tree of Perdition + Aquamoeba to kill with commander damage. Add Cephalid Inkshrouder for unblockable.
- Magistrate's Scepter + ways to add charge counters for infinite turns. The actually requires a large number of cards caged to pull off.
- 2-3 charge counters + Empowered Autogenerator/Astral Cornucopia + Quicksilver Elemental + Leech Bonder/Farmstead Gleaner/Pili-Pala. The mana can be Everflowing Chalice with Pili-Pala.
- Aetherling + Perilous Vault for there to be only Mairsil and lands. Stacking the blink first allows the resolution to find Mairsil in exile at the end of the turn.
- Lotus Bloom + Endling + Cinderhaze Wretch/Leech Bonder + haste
- Lotus Bloom + Pili-Pala + Cauldron of Souls + Carnifex Demon + haste
- Lotus Bloom + Leech Bonder/Farmstead Gleaner + Cauldron of Souls + haste
- Soldevi Adnate + Cinderhaze Wretch + any mana rock + Corpse Dance. The uptap can be Pili-Pala or Leech Bonder if the mana rock taps for .
- Empowered Autogenerator/Astral Cornucopia/Everflowing Chalice + Spark Double/Sakashima the Impostor + Tawnos's Coffin + Farmstead Gleaner/Pili-Pala = reset Mairsil without losing charge counters.
- Mirror Gallery/Spark Double + Myr Propagator + Training Grounds + Cinderhaze Wretch + mana rock +haste = infinite token Mairsil. This gets more simple when Cryptic Trilobite is added.
- etc...
I'm always looking for good activated abilities on terrible cards, or new synergies with counters.