This just happened to us as well. Only difference is it was Yaggy and Fader that didn't spawn properly and we cannot hit Fader through the wall. I'm pretty sure it's because Kall got pushed too close to the wall before he encased himself in ice.
https://steamcommunity.com/sharedfiles/filedetails/?id=3823336206

hammer work on this buged fight.

I ran into this bug today. The Fader is hovering high up behind a wall; even with lightning spells or a two-handed club, my attacks don't land.

I've just tried the fight. After initially defeating the boss, The ghost bosses didn't spawn in the cave, so I went outside and the stag boss was there just by the entrance. I beat him in the snow, then all the rest of the ghost bosses spawned outside too, just outside of the arched portal. After defeating them all, I went back in the cave and the last boss was encased in an ice ball and I couldn't do any damage to it to release him. So I'm stuck and unable to complete the game.

I just reloaded the last world save and found myself back in the boss cave at the start of the fight. (That's not where I last saved, but I guess the game did that) I fought through rounds 1 and 2 again. This time all the ghost bosses did appear inside the cave with me as expected. But I noticed I had no rested bonus. The 3rd and final iteration of the boss did appear this time, so after a few minutes trying to fight it, I stupidly decided to leave the cave, get some rested bonus and repair my gear. I went back to the boss portal and was locked out. I guess I'm going to have to go and stop 3 more Jotan invasions and get some more coals to get back in there now. What a complete waste of time this game is because of all the bugs.

Sorry to hog the thread, but I thought I'd share my ending gameplay: I loaded into the game and took the portal back to my base to see if the coals in my inventory had reappeared - I had previously reloaded the world save to try and fix the bugged boss fight, so maybe they'd reset? This would save me dealing with Jotan invasions three more times to get those coals again.

Fortunately, the boss portal coals were there in a chest, so I took them and went back to the final boss portal to try and get back in. When I did, the boss was in his third phase, just where I'd left him before getting locked out. I managed to kill the final phase of the boss and he dropped the 2 final items. I was so relieved!

I thought I'd have an easy boat ride back to base, then walk back to the henge to place the final item and see the game end credits. My Ashlands big strong boat was moored at the Deep North coast nearby, on an ice shelf. So I skidded about on the ice shelf, jumped in the water and climbed in the boat. I carefully reversed it out into the sea, but I gently nudged an iceberg and the boat instantly broke. It seems while it had been resting there throughout my Deep North play-though, it's health had reduced down to 1 tick. I hadn't placed a workbench when I arrived and just assumed the boat would have enough health to sail home. Although, in my excitement to end my playthrough and enjoy the gentle cruise back to base, I forgot to check. So there I was in the water, unable to climb back onto the ice shelf, with endgame items on me. I knew if I died, I would never be able to retrieve them.

I admit I just reloaded an old save until I was somehow standing with my endgame items on the shore again. I can't help but think I was either unlucky, or just very bad at the game. I do blame bad/flawed design and poor bug-fixing, of course. I'd previously had world saves and backups deleted mysteriously in the 1.02 update which permanently lost my Dyrnwin flaming sword, so my confidence in the game was shot. Anyway, the final biome and boss fight nightmare is over now, so I can finally move on.

I gathered the offerings and summoned the boss again. When I killed the "Lost Aspect," the first one emerged from stasis—so I ended up fighting two bosses at once! It was quite fun; I only died 6 times. At the end, Hugin didn't show up—is this another bug? Last time I played through on Normal difficulty, both ravens appeared, but now only Munin is there.

Hugin was probably too embarrassed about all the bugs to show his beak.