The Descent: Journeys in the Dark (Second Edition) Wiki
Strength In Numbers
0 Threat to Purchase
Exhaust this card when a SMALL monster performs an attack, before dice are rolled. This attack gains:
Pierce X, where X is the number of monsters from that monster's group adjacent to the target.
First Legion 1/12 1 Threat

Essentially, this card rewards you for leading your small monsters to attack the heroes in large groups.[1]