Thelon's Chant is a uncommon Enchantment card from Fallen Empires (FEM), collector number 76. Current market price: $0.17, TCGplayer market prices via Scryfall.
At the beginning of your upkeep, sacrifice this enchantment unless you pay {G}. Whenever a player puts a Swamp onto the battlefield, this enchantment deals 3 damage to that player unless the player puts a -1/-1 counter on a creature they control.