Soulherder is a uncommon Creature — Spirit card from Modern Horizons (MH1), collector number 214. Current market price: $1.81 (foil $10.27), TCGplayer market prices via Scryfall.
Whenever a creature is exiled from the battlefield, put a +1/+1 counter on this creature. At the beginning of your end step, you may exile another target creature you control, then return that card to the battlefield under its owner's control.