Dementor

When Dementor blocks or becomes blocked by a creature, that creature gets -1/-1 until the end of
turn
Dementors are among the foulest creatures that
walk this earth. They infest the darkest, filthiest places, they drain peace, hope, and happiness out of the air around
them... Get too near a Dementor and every good feeling, every happy memory will be sucked out of you.
If it can, the Dementor will feed on you long enough to reduce you to something like itself... soulless and evil.
-Remus Lupin