Mob Mentality is a uncommon Enchantment — Aura card from Visions (VIS), collector number 88. Current market price: $0.31, TCGplayer market prices via Scryfall.
Enchant creature Enchanted creature has trample. Whenever all non-Wall creatures you control attack, enchanted creature gets +X/+0 until end of turn, where X is the number of attacking creatures.