Pack Mentality is one of the two researchable abilities for the Crawler, the main early game Chaos unit. Its great ability and low cost make it a must-buy for large groups of Crawlers.
Upon researching the ability, Crawlers will receive an increase in damage scaling up to a pack size of 5. This makes crawlers in groups gain individual damage until the group has 6 crawlers in. This makes them able to dive in and execute low health units from the bonus damage, and to trade well with melee units. It's a very useful ability to chase down and kill weaker units like Earths, Swordwrath, and Archidons, and frequently forces an Order or Elemental player to fall back to protect their low hp units, but it would be hard for crawlers to kill against a more tanky Armoured units or AoE, like a Charrog without heavy losses. This is why the Crawlers' other upgrade, Predatory edge, is a great asset and combination towards Crawlers. Usually players would research both abilities at the same time.