Bitter Feud is a rare Enchantment card from Commander Anthology Volume II (CM2), collector number 84. Current market price: $0.64, TCGplayer market prices via Scryfall.
As this enchantment enters, choose two players. If a source controlled by one of the chosen players would deal damage to the other chosen player or a permanent that player controls, that source deals double that damage to that player or permanent instead.