Thoughtweft Imbuer is a uncommon Creature — Kithkin Advisor card from Lorwyn Eclipsed (ECL), collector number 38. Current market price: $0.15 (foil $0.25), TCGplayer market prices via Scryfall.
Whenever a creature you control attacks alone, it gets +X/+X until end of turn, where X is the number of Kithkin you control.