Bone Rattler is a rare Creature — Skeleton card from Mystery Booster Playtest Cards 2019 (CMB1), collector number 35. Current market price: $0.57, TCGplayer market prices via Scryfall.
When Bone Rattler is put into your graveyard from anywhere, exile it. When you do, create four Reassembling Skeleton token cards and put them into your graveyard.