AEtherling
: MVP All star here. This should never not be your first cage target unless an opponent somehow gets rid of it.
Anger
: The best haste enabler in the deck. You usually want to try to bin this as quickly as you can unless you suspect grave hate is coming your way.
Arcanis the Omnipotent
: Card draw on a stick. His second ability is OK at best, but you’re only ever using his first to refill your hand.
Cavern Harpy
: This lady is never a bad cage target. Being able to bounce Mairsil back to hand at instant speed for only 1 life is well worth having to recast him in many situations.
Doom Whisperer
: This gem from GRN has a few uses. It can serve as an undercosted beater (with no friggin' downside! What the hell, WOTC?), or as a combo enabler (use it's ability multiple times to just dump your library to your GY and find whatever you need to cage). Its stats are good enough that I don't mind hardcasting it instead of caging it. Either way, the card selection is real.
Dralnu, Lich Lord
: Dralnu has one of the best abilities on one of the worst creatures. Fortunately, the downsides don’t matter when Mairsil has him caged up! One of my favorite lines of play is to play
Buried Alive
/
Intuition
to dump
AEtherling
, Dralnu, and typically
Anger
into the GY before playing Mairsil on my next turn. You play him, cage Aetherling, then flicker him and cage Dralnu. Then you can give flashback to the Buried Alive/Intuition to find whatever you need to go from there.
Hateflayer
: Not very useful on its own I admit, but this will flat-out kill an opponent when combined with the Tree.
Knacksaw Clique
: Even if you’re not using it to combo out, it’s still a nice ability. Deck theivery is always a fun option.
Kozilek, the Great Distortion
: I like Big Daddy Koz in most parts of the game. If you do somehow get to the late game and can cast him, you’ve got a solid finisher set up. If not, caging him on to Mairsil gives you essentially multiple
Force of Will
’s in the deck (plus you can cage what you discard later!)
Memnarch
: Purely there for the combo, though stealing an opponent’s Sol Ring or
Blightsteel Colossus
can be nice.
Morphling
: I partly wish Mairsil was 5-color, simply so we could make this a X-ling tribal deck. The original X-ling creature, Morphling has several uses. Primarily, you want to use it as an untap effect for Mairsil, along with a protection piece, giving him shroud when needed. You can use the other abilities to Voltron it up in grindier games, but I’ve rarely needed them for that.
Oona, Queen of the Fae
: Primarily a combo piece, but she’s great at building up a boardstate if you’ve looking at a full board on your opponent’s side, particularly against mono-colored decks.
Pack Rat
: This Draft Nightmare is fantastic at giving Mairsil more cages. Oftentimes you can just discard the card you want to cage. One of the better options early-game.
Pili-Pala
: Basically here for more infinite mana shenanigans. You tap Mairsil for Lotus mana, spend a mana for a quicksilver activation, then spend the other two to untap him, netting a mana and going infinite.
Quicksilver Elemental
: If Aetherling gives you the setup, QE gives you the payoff. So many ways to use this little buddy to go infinite. Usually my second tutor target after Aetherling.
Razaketh, the Foulblooded
: This guy’s an absolute beast. To be fair, you won’t always have a board state, but there are enough token generators in the deck that it’s worth having Razzie in here as a payoff.
The Scarab God
: This is one of maybe a small handful of these cards I don’t mind hardcasting, as those abilities are just brutal. It’s great when caged on to Mairsil, but even if not, it’s a very hard-to-remove threat that’s worth running.
The Locust God
: So the activated ability is not amazing I admit. But it has a few uses. First up, there's enough card draw in here to get a decent bit of bodies out of this guy. Bodies I can then either use for attacks, blocks, or sac fodder for Razaketh. Plus if I can draw my deck with him out, it's yet another way to close out the game from that moment. This is thoroughly a flex spot for me and I've no guarantee I'll keep this, but it's in here for now.
Torchling
: See Morphling, but red.
Tree of Perdition
: Even without the Hateflayer kill, dropping someone to 4 life is just demoralizing and wrong and beautiful.
Viashino Heretic
: Another flex slot, I simply run this because it’s artifact removal with a little bit of burn added on. It’s not a combo piece, but you caaaan do some interesting things with it and Memnarch. - UPDATE: This poor little guy is dead and gone. Now that I have a Walking Ballista, I don't care too much to keep this one around. It was fun while it lasted, little guy!
Walking Balista: This multi-format all star has plenty of uses, but is primarily an infinite mana sink for the win. Whether it's cast on its own or caged by Mairsil, this guy ends games.