Promise of Loyalty is a rare Sorcery card from Marvel Super Heroes Commander (MSC), collector number 142. Current market price: $0.23 (foil $0.41), 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.