Guide of Souls is a rare Creature — Human Cleric card from Modern Horizons 3 (MH3), collector number 29. Current market price: $3.54 (foil $4.16), TCGplayer market prices via Scryfall.
Whenever another creature you control enters, you gain 1 life and get {E} (an energy counter). Whenever you attack, you may pay {E}{E}{E}. When you do, put two +1/+1 counters and a flying counter on target attacking creature. It becomes an Angel in addition to its other types.