Grand Ossuary is a common Plane — Ravnica card from Planechase Anthology Planes (OPCA), collector number 31. Current market price: $1.89, TCGplayer market prices via Scryfall.
Whenever a creature dies, its controller distributes a number of +1/+1 counters equal to its power among any number of target creatures they control. Whenever chaos ensues, each player exiles all creatures they control and creates X 1/1 green Saproling creature tokens, where X is the total power of the creatures they exiled this way. Then planeswalk.