Krosulhah is a named dragon from Dragonborn, and one of the most outspoken servants of Miraak on Solstheim. His name reads “Sorcerer–Day–Mind” (the last word can also be read as “Will”). Before attacking, he lets you know that Miraak is growing stronger and has ordered your death, so there is never any doubt about whose side he is on.
Where to find them
Krosulhah turns up outside Nchardak, the flooded Dwemer ruin, once you have completed The Path of Knowledge. That puts the fight right after a long dungeon crawl, so be ready the moment you step back outside.
Notes
Krosulhah ranges from level 30 to 50, with 1860–3071 health, 150 magicka and 197–264 stamina, making him a serious opponent compared with the dragons of early Skyrim. When he falls there is a chance that Miraak shows up and absorbs the dragon soul before you can, a recurring trick in the Dragonborn questline.
He is also one of the named dragons that can be tamed with the complete Bend Will shout. Even under your control he keeps praising his old master, predicting that Miraak will return and reclaim his kingship over Solstheim; those lines stop once Miraak has been defeated. Hit him with Dragonrend and he reacts with outrage, calling the shout foul and a treachery no true dragon would stoop to.
His remains yield Dragon Bones, Dragon Scales, gold and a random mix of enchanted or plain items. Those bones and scales are worth holding on to for dragon armor, and the encounter makes a good warm-up for the larger confrontations at the Temple of Miraak later in the story.
