Howlers are pack-hunting predators from Pandemonium, a peril suitable only for top-tier adventurers to deal with. That’s because, according to the lore in Mordenkainen’s Tome of Foes, these CR 8 fiends never appear alone. Either they’re accompanied by others of their kind, or they’re trained to the service of a more powerful fiend or other evil master and fighting by its side.
Theirs is a nasty ability contour: very high physical ability scores across the board, with an extraordinary peak in Wisdom. This outlier score both powers their Mind-Breaking Howl action and gives them keen hunting senses. (Although it’s not listed in the Mordenkainen’s errata, with a Perception modifier of +8, their passive Perception should be 18, not 15, as published.) But since they lack spellcasting ability, their primary attack is Strength-based, and their top non-Wisdom scores are Strength and Dexterity, I’m going to classify them as shock attackers. Move fast, hit hard.
Howlers have darkvision and therefore attack between dusk and dawn. They’re resistant to cold, fire and lightning damage on top of physical damage from nonmagical attacks, so they’re not going to pay particular attention to a spellslinger who doesn’t think outside the box. Acid or thunder damage will get their attention, but necrotic, radiant, psychic or force damage is what really draws their ire. (Shout-out to James Haeck for laying out the tiers of damage types in a way I hadn’t actively considered before in his editor’s note on this D&D Beyond post by Melissa “MellieDM” Doucette.)
A fiend with trained howlers by its side always travels with at least two, if not more; an independent pack of howlers comprises no fewer than four, allowing them to split up and still use Pack Tactics against two targets if need be. Still, be very careful about sending more howlers than party members at your player characters, because one howler is enough to put up a decent fight against even a 20th-level PC. I’d place the ceiling at one howler per PC, plus one additional howler per magic weapon, and that’s assuming they’re the only foes on the field.
Despite their Wisdom score, howlers are bloodthirsty creatures that won’t shy away from a fight if they have any chance of victory. However, they do have a nose for target selection. My stock formula for a predator is “the young, the old, the weak, the isolated and the oblivious,” and these criteria are their starting point, to which they add one other: the frightened, i.e., those who’ve failed their saving throws against Mind-Breaking Howl. They deviate from these criteria only when they take damage of one of the types listed above.
Mind-Breaking Howl is a recharge ability—in other words, rationed, and therefore the powerful action they should lead with. But since Mind-Breaking Howl recharges only on a roll of 6, it’s most likely a once-per-encounter ability. Being pack hunters, howlers have an instinctive understanding that they shouldn’t all use it at once. Maybe, in fact, their dominance order is also the order in which they use their Howls.
The effect lasts only until the end of the howler’s next turn, and a successful saving throw confers immunity, so the idea is to try to use it repeatedly against those opponents who fail their saves. Once every opponent has made their save, there’s no more point in taking the Mind-Breaking Howl action, but I’m not sure howlers have the Intelligence to recognize that it’s no longer having an effect; I think it’s practically a reflex. If a howler’s Howl recharges, it uses it, unless there’s a higher-status howler whose Howl has also recharged, in which case it defers for the current turn. Before using Mind-Breaking Howl, a howler instinctively positions itself to encompass at least six opponents—or every opponent, if there are six or fewer—within its conical area of effect.
When Mind-Breaking Howl is on cooldown, or when it’s not a particular howler’s turn to use it, howlers Multiattack, always attacking in concert with one another to take advantage of Pack Tactics. They’re uncommonly tenacious: If they’re tearing apart a victim, running up and hitting them won’t make them stop, unless the attacker wields a magic weapon or, as mentioned before, deals one of the types of damage that cheeses them off. In this case, the one struck will turn against whoever attacked it, as will one other. Any howler remaining continues to attack its chosen victim unless it’s the only one left doing so, in which case it joins its fellows.
Their truculence is so intense that it overcomes their self-preservation instinct as long as they have even one ally present. Only when a howler is the last creature standing on its side does it retreat, and even then, it retreats only if it’s at least moderately wounded (reduced to 63 hp or fewer). It uses the Dash action as it flees—unless its Mind-Breaking Howl happens to recharge, in which case it wheels around and uses that action against its pursuers (whether or not they’ve all succeeded on their saves against it) before resuming its exit.
And that’s going to be it for Mordenkainen’s Tome of Foes for the time being. The next creature I’m going to look at is the bone knight from Eberron: Rising From the Last War. Look for other monsters from Mordenkainen’s in The Monsters Know What They’re Doing, Vol. 2, tentatively scheduled for release in November 2021.