9:32 - "But if you pay the one blue mana again, you can then refresh those abilities and reset it for one more use." - You actually undersold the Quicksilver Elemental interaction because each activation doesn't just give you one more use but rather exponential gain. This works whether Quicksilver is targeting Mairsil or caged by him. Let's say Mairsil has three abilities - A, B, and C (the last being Elemental). If you activate C, Mairsil now has AABBCC (with one C having been used up for the turn). But if you activate it again (with the unused copy of C), it doubles all abilities, including the copies, so Mairsil now has AAAABBBBCCCC (with 2 Cs used and 2 Cs unused). Activate a third time, and you've got 8 copies of each ability.

thank you for this video. mairsil is my favorite commander due to the ridiculous amounts of ways you can build the deck. sadly currency converter and bag of holding don't work with mairsil since their abilities are linked. this deck is absolutely bonkers if you build it with a ton of looting effects, one of the best among them is civilized scholar, since it usually untaps itself. endling is amazing because when combined with cards like glen elendra archmage or bloodfire colossus/kagemaro, first to suffer and crystalline crawler it kinda softlocks the game. skywing aven and cavern harpy work both as protection and to recast mairsil. mausoleom wanderer and kozilek, the great distortion work great as control options. and i'm particularly fond of trench behemoth and cinderhaze wretch as "free" untappers to keep abusing tap abilities like galecaster colossus.

Unfortunately I don't think Bag of Holding and Currency Converter's second activated abilities work. One of the official rulings states "If Mairsil gains an activated ability that's normally linked to a non-activated ability of the card it came from, the ability Mairsil has isn't linked to any ability. For example, if Mairsil exiles Prototype Portal, the activated ability creates no tokens at all; it doesn't create Prototype Portal tokens." Since the second ability of each card is linked to a non-activated ability, they would fall under this clause and only the first activated ability could be used.

I love Mairsil. I've been brewing him for about 6 months now. While I agree with most of your card choices here, I'd like to point out that Tree win con is pretty bad. It was originally my win con of choice as well, But you can free up 3 card slots if you just run Death Cultist instead. Death Cultist has "Sacrifice Death Cultist, target player loses 1 life and you gain 1 life". This is easily abusable by caging Endling + Death Cultist + Crystalline Crawler. Pay one black mana to give Mairsil undying, sacrifice Mairsil for Death Cultist ability, ping for 1, Mairsil comes back with a +1/+1 counter on him you can activate Crystalline Crawler to add one black mana to your mana pool, then repeat the process for infinite damage. Mairsil also has access to a unique, and powerful likne. Mirror Mad Phantasm. You only need 1 generic and 2 blue mana for this combo, MMP caged, and another creature on board. Mairsil activate MMP ability, you put him into your commander zone rather than tucking him, the deck mills out due to not finding MMP. With your deck in the graveyard you can now get Nacromoba from the GY to the battlefield, pay one blue to unearth Fatestitcher, and now you can cast Dread Return for it's flashback of sacrificing three creatures to reanimate Thassa's Oracle for the win. I adore this commander and how unique, and versatile it is. He packs a ton of resilience and is getting new cards with every set that can be caged up!

You can't return cards exiled with mairsil with any hability of a caged artifacts or creature. Those habilities are linked so they only refer to cards exiled by habilities PRINTED on the same card. Unfortunately doesn't work.

Mirror of Fate would be interesting to cage in this deck. It would definitely work best with haste enablers and undying effects.

certanly an interesting list you got here but you are missing a personal favorite of mine thats not only really spicy but lets you make one of my favorite jokes and thats pack rat..cause despite all my rage im still just a pack rat in a cage. anyway cloning marasil even when you can actively break the legend rule is powerful and even though you need to discard to activate pack rats ability it aint so bad if you were plkanning on caging whatever you pitched to the rats ability. as a result this rat can make marsil a pesky card to remove cause you simply replace him with another before the spell removing him finishes to resolve

Watch out with both Bag of Holding and Currency Converter! You can only bring back cards exiled with this instance of the card. If Mairsil is removed from play in any way (such as blinking with Aetherling) he is a new instance and no longer sees the cards exiled with it previously. So these cards do not let you grab any card with a cage counter from exile, it just lets you pick those you exiled from the moment he last entered play.