Gyruda, Doom of Depths

The companion mechanic asked a deckbuilding question, and this one asked the strangest: can you build a functional deck where every mana value is even? Companion promised an extra card from outside your starting deck in exchange for a construction tax, and the even-only restriction is the harshest in the cycle, because it deletes odd-cost roleplayers wholesale and leaves you building around twos, fours, and sixes with nothing at one, three, or five. What paid for that pain was a payoff no other companion offered: a body that reanimates. The mill-four-then-reanimate trigger turns this into a repeatable cheat-into-play engine, and the same even-value constraint that shapes the deck also stacks the yard with legal targets, since every creature you own is guaranteed to qualify. That closed loop, restriction feeding payoff, is why it spawned genuine combo lines rather than goodstuff piles: flicker or blink the Demon Kraken and you mill and reanimate again, and with the right even-cost creature waiting the loop becomes a kill.

Companion as a mechanic was reworked shortly after it debuted, walking back the "always available for free" tempo that broke multiple formats; the added activation cost changed how every companion played. This one survived that change better than most, because its power was never the free access alone but the reanimation engine bolted to a 6/6, which the errata left untouched.