Cloudkill

Fading 4 (This enchantment enters the battlefield
with 4 fade counters on it. At the beginning of your
upkeep, remove a fade counter from it. If you can't,
sacrifice it.)
At the beginning of combat on each player's
turn, put a -1/-1 counter on each creature
controlled by the active player.
"Acidity level required for humanoid dissolution
remains... inconsistent." -Doomsday, lab notes