Soulherder is a rare Creature — Spirit card from Secret Lair Drop (SLD), collector number 2174. Current market price: $7.01, 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.