Vsara, The Corrupter

Protection from creatures with betrayal counters
on them
At the beginning of your upkeep put a betrayal counter on each creature your opponent's control, then take control of all creatures with 3 or more betrayal counters on them, they lose all colors and creature types and become become Black and Phyrexian Horrors. At the start of your opponent's upkeep, they take 1 damage for each betrayal counter on creatures they control.