Q: What happened?
A: When you play a minion, the On Play Phase resolves (including all death processing) before the Battlecry Phase begins. Illidan Stormrage is an On Play Phase trigger, allowing us to kill an Anub’ar Ambusher which returns Twilight Drake to the hand. Then the Battlecry Phase begins, and despite being in the hand, Twilight Drake triggers and gains 10 Health (because there are 10 cards in our hand) — twice, because Brann Bronzebeard is still in play.
Q: Why does the 4/21 Twilight Drake retain its Enchantment when being played?
A: Bolvar Fordragon is another card that can build an Enchantment in the hand and retain it when being played. Battlecries being able to resolve in the hand is the bug, but this part is working as intended.
Q: How hard was this to pull off?
A: 1/6 chance every time we did the setup. (We got it first try.) To damage all minions to correct health, we used Lorewalker Cho and traded 1, 2 and 3 damage spells back and forth.
Q: Is this the theoretical maximum?
A: 4/41 is the theoretical maximum. (To do this, the second time you play Twilight Drake, a card has to be drawn before the Battlecry Phase, such as by using the setup of the 4/11 Twilight Drake video: https://www.youtube.com/watch?v=ZKf8WId7N0Q )
(Alternatively, there’s a trick with Mind Control that lets you make your hand have 11 cards at once, discovered by HysteriA: https://www.youtube.com/watch?v=8GbKPLvUHZk but it would not let you get more than 4/41 — the first play, Twilight Drake can’t be bounced back into a hand of 10 cards, and the second play, Twilight Drake would be one of your 11 cards, so it would just be an alternate way to have 10 cards in your hand. You also can’t reach 12 cards in hand at once, because to do the Mind Control trick again you must play one of your cards.)
Patch: 22.214.171.12433 (LoE patch)
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Music used in this video:
Patashu — Satellite 2 Satellite https://soundcloud.com/patashu/satellite-2-satellite
Patashu — OHC 342 Grand Opening http://compo.thasauce.net/files/Patashu_-_OHC_342_Grand_Opening(OHC342).mp3
Image credit: http://s341.photobucket.com/user/Crisis-The-Striker/media/BiggerThanBig/930_maxs_zps7ca2551a.jpg.html