Necrosynthesis is a uncommon Enchantment — Aura card from Innistrad: Midnight Hunt (MID), collector number 115. Current market price: $0.18 (foil $0.36), TCGplayer market prices via Scryfall.
Enchant creature Enchanted creature has "Whenever another creature dies, put a +1/+1 counter on this creature." When enchanted creature dies, look at the top X cards of your library, where X is its power. Put one of those cards into your hand and the rest on the bottom of your library in a random order.