There are plenty of challenging bosses in Diablo 4 but one early contender in particular has really been putting players in their place. If you want a real challenge to throw your latest bit of build theorycrafting at, I've found a really angry werewolf that would love to meet you.
The Den Mother is an enormous lycanthrope with loads of AoE, stuns, and long range melee attacks who regularly summons her children (that is, 8 foot tall werewolves) to aid her. She turned me into absolute mincemeat when I first innocently discovered her, and though she's not quite the Butcher, she certainly demands a bit of strategy to beat.
Where to find the Den Mother
If you do fancy your chances against the furry matriarch, first head to the village of Margrave. It's a village south of the hub city of Kyovashad—just walk out the east gate and then go directly south to reach it. There, you can pick up the Legacies of Light's Watch sidequest. It's not necessary to reach the Den Mother, but it'll give you some extra rewards and lore for exploring her domain.
The quest will send you directly north of Margrave, to the Light's Watch dungeon. It's a pretty big place, with an interesting theme—and quite a few tough minibosses to beat before you're given an audience with the Mother herself. Keep in mind that like all the optional dungeons, you need to finish it all in one go—you're allowed to portal back to town for supplies if you need to, but if you leave the dungeon and go and do something else, your progress will be lost.
Tips for fighting the Den Mother
Your choice of class will make a big difference here—she's a particularly good counter to Barbarians, who'll struggle to get up close to her without burning through health potions. Heroes with more ranged options will fare better. Sorcerers can probably kill her in five seconds flat, but we all know they're OP.
Make sure you keep out of the shadowy pools on the ground, save your dodges for her very telegraphed but very long range swipe attack, and manage your potions well, keeping in mind she'll drop more at each increment marked by an arrow on her health bar. It also won't hurt to upgrade your health potion for that extra edge.
You'll spend a lot of this fight dodging and running away from her claws, so invest in skills that apply damage over time like bleeding or fire. Barrier skills like Iron Skin for barbarians will also help absorb her slashes—but save them for when you really need them. If you're still struggling, come back at a higher level—it's easy to reach her around level 10, but 15+ may be wiser. Though she will scale to match you, she's a little easier once you've unlocked more abilities and earned some legendary gear.
Once you beat her, she'll drop some tasty loot—but you'll also unlock an Aspect for all your characters, the Aspect of Conflagration. This buffs the Incinerate spell for Sorcerers, once it's infused into an item, so if you're keen to try out a fire build it'll be useful. If not… well, just think of it as your trophy.
Good luck out there, and watch out for the full moon.
Diablo 4 Sorcerer build: Element-ary
Diablo 4 Barbarian build: Stay buffed
Diablo 4 Rogue build: Need for bleed
Diablo 4 Druid build: Air and weres
Diablo 4 Necromancer build: Stay undead