Promise of Loyalty is a rare Sorcery card from Murders at Karlov Manor Commander (MKC), collector number 79. Current market price: $0.28, TCGplayer market prices via Scryfall.
Each player puts a vow counter on a creature they control and sacrifices the rest. Each of those creatures can't attack you or planeswalkers you control for as long as it has a vow counter on it.