Pack Tactics. A howler has advantage on attack rolls against a creature if at least one of the howler's allies is within 5 feet of the creature and the ally isn't incapacitated.
Multiattack. The howler makes two Rending Bite attacks.
Rending Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit:10 (2d6 + 3) piercing damage, plus 22 (4d10) psychic damage if the target is frightened. This attack ignores damage resistance.
Mind-Breaking Howl (Recharge 4–6). The howler emits a keening howl in a 60-foot cone. Each creature in that area must succeed on a DC 13 Wisdom saving throw or take 16 (3d10) psychic damage and be frightened until the end of the howler's next turn. While a creature is frightened in this way, its speed is halved, and it is incapacitated. A target that successfully saves is immune to the Mind-Breaking Howl of all howlers for the next 24 hours.
Why does the howler sing? Doing so causes its prey to flee, and surely stealth would make for better hunting in howling Pandemonium. There is only one answer: the creature can taste fear.