It could be to pay a cost, or maybe because the effect of a spell or ability tells you to
Exile this creature at the beginning of the next end step, then you may cast it from exile on a later turn.) When this creature enters, search your library for a basic Plains card or a creature card with mana value 1 or less, reveal it, put it into your hand, then shuffle
They award two Prize Cards if defeated
If they had succeeded, you would have become like they are, only weaker and under their command." Gandalf Illustrated by: Joseph Meehan Multiverse ID: 621318 Not Legal Traditional Standard Not Legal Traditional Alchemy Rulings 2021-03-19 If an ability triggers whenever an opponent loses life and causes you to gain life, such as the ability of Exquisite Blood, this will loop until either you win the game or a player takes an action to break the loop
Screaming Nemesis Screaming Nemesis has to be one of the most hyped cards from Duskmourn