Rot Elemental

Menace.
When Rot Elemental enters, tap it and target creature. Rot Elemental and the targeted creature do not untap as long as at least one of them is in play
At the beginning of your upkeep, put a -1/-1 counter on the targeted creature and a +1/+1 counter on Rot Elemental. This ability only triggers if both creatures are in play and tapped.