the adversary page says
Overwhelm - Reaction: When the Zombies mark HP from an attack within Melee range, you can
mark a Stress to make a standard attack against the attacker.
but when I open the sheet this shows up:
Overwhelm - Passive: When a target this adversary attacks has other adversaries within Very Close
range, this adversary deals double damage.
Zombie legion also appears to have two different descriptions for Overwhelm.
Rotted Zombie has one that says 2 dmg and the other says X dmg under Group Attack.