Theres the usual balance considerations of other formats, but also things like whether or not a card promotes unhealthy play patterns for a four-player casual format
(To mill two cards, a player puts the top two cards of their library into their graveyard.) The card you put on top of your library, if any, may be one you just milled, or it can be one that was already in your graveyard
For example, if a single-faced card is a copy of The Legend of Kyoshi, the chapter III ability will cause it to be exiled and then remain in exile
Creature - Goblin Warrior Color tag templates(None) Kiki-Jiki, Mirror Breaker Haste : Create a token that's a copy of target nonlegendary creature you control, except it has haste
Its often the ultimate companion piece for plenty of outstanding partner commanders who can draw into it or cheat it out, and is essentially the final boss of blue magic that can turn every card in your hand into an unstoppable threat