Most Strongholds in Diablo 4 are pretty straightforward, with a few clearly defined objectives in neat order. The Onyx Watchtower is a bit more complicated than others. On the bright side, it's only moments away from the town of Ked Bardu in the Dry Steppes, and you unlock a dungeon for conquering it.
Besides the Onyx Watchtower's narrow, winding paths, you also have to deal with a couple of different locked doors. Bandits in Diablo are typically quite crafty like this. Here's how you can overcome their schemes and clear the Onyx Watchtower with ease.
Slay The Captain's Lieutenants
The first objective for this Stronghold actually involves destroying objects and defeating the general groups of bandits. But that's easily done while pursuing the Lieutenants. Just be sure to destroy any supply carts, tents, or support beams that you come across.
Once you enter the Onyx Watchtower, you should go over to the western side to fight Crocus the Greedy. He'll drop the Storeroom Key that will unlock the gate over on the eastern side, which leads to another Lieutenant and a group of bandits. None of these Elites are particularly dangerous. After clearing the Storeroom, go back west to climb the watchtower.
Thidrek, the Beast-Master, is one of the tougher enemies in the Onyx Watchtower. He has a devastating knockdown ability in addition to poison. But the real threat is that he keeps Elite beasts in cages around him. If you can help it, do not break the cages while fighting Thidrek. Use any Crowd Control moves you have to get the Beast-Master away from his captives.
The final Lieutenant to fight is Talida. She's quite deadly with her Multishot move and paired with her Mortar ability, she has excellent attacks for both long and close range. Fortunately for us, she's just not very tough. Evade her attacks until you can unleash all of your own most powerful moves.
Slay Captain Ezmin
After defeating the last of the Lieutenants, the gate at the center of the Onyx Watchtower will open automatically. In the alcove beyond, you'll find Captain Ezmin waiting for you. She has some pretty strong mortar attacks, but her true power is repeatedly summoning hordes of bandits.
This is actually a really easy boss fight compared to the others in Diablo 4. Ignore the endlessly spawning bandits and keep attacking Captain Ezmin. Avoid the burning rings that she summons on the ground at all costs. If you do feel like taking out some of the additional bandits during the fight, then at least focus on the Firebrands. That will give you the most free time to take down the boss herself. Then turn in the conquest and unlock a well-earned Renown increase.