Keen Hearing and Smell. The weasel has advantage on Wisdom (Perception) checks that rely on hearing or smell.

Attach

A dire weasel that hits with its bite attack latches onto the opponent’s body with its powerful jaws. An attached dire weasel loses its Dexterity bonus to AC. An attached dire weasel can be struck with a weapon or grappled itself. To remove an attached dire weasel through grappling, the opponent must achieve a pin against the creature.

Blood Drain

A dire weasel drains blood for 2d6 points of Constitution damage each round it remains attached.

Pack Tactics. The hound has advantage on an attack roll against a creature if at least one of the hound's allies is within 5 feet of the creature and the ally isn't incapacitated.

Bite. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit:  2d6+3 piercing damage.