Xolphnurth the Sorceror

Whenever an opponent either discards a card or
puts a card from their library into their graveyard
you may choose one:
{u}: Draw a card
{b}: Creatures your opponents control gain -1/-1
until the beginning of your next end step
{u}{u}{b}{b}: You may cast target creature spell in
an opponents graveyard with converted mana
cost 5 or less without paying its mana cost