I'm not sure about the costs, but those can always be tweaked.

Mmm... it takes a while to go off; tolet you bypass casting costs on a load of (presumably huge) stuff..

I think I like it. It's a great big "I win" clock; the opponent can see it coming and deal with it.

Hee. Yes, this looks like a recipe for fun. (Or card disadvantage. One or the other.) Also requires a deck with a load of legends in it, which makes me think of Captain Sisay, which in turn makes me wonder if this could itself be legendary?

Ah, yes. Captain Sisay. I wish I had the money and drive to build a beautiful EDH deck around her. For her sake, this should indeed be legendary.

Edit comment was supposed to read "It's already hard to pull off."

Oooo. Hexproof changes the card significantly for the better.

(I do have an EDH deck for her. It's lots of fun, especially cards like Sword of the Chosen and Day of Destiny. The two nonlegendary creatures in the deck are Kitsune Healer and Konda's Hatamoto :) )

Hexproof does make it better. However, you're already talking a six-turn countdown (barring untap shenanigsns), during which you have to exile six different cards (if you care about the Legend Rule and aren't using Brothers Yamazaki as two of your cards), which, if this doesn't go off, creates a lot of card disadvantage. I've almost convinced myself that this could cost less, even.