They stay in your sideboard until you grab them with a learn effect, or another kind of card that could look for them outside the game
Note, all prices were taken from TCGplayer on September 16, 2024
The team even considered making cards based on specific moves or iconic things that werent characters themselves (for example, something like a Round One or "Perfect" card), but that they instead decided to represent moves as abilities on each creature
January 6, 2014 11:47 a.m
Pestilent Souleater is perfect for making Mairsil much more of a threat both in creature combat and against opponents, and I like that its ability is relatively cheap on mana or even free if you want to go the Phyrexian mana route and pay life
More importantly, since it exiles the creature (or planeswalker) it targets as well, it is often a lifesaver against hard-to-kill threats like Koma, Cosmos Serpent