tl;dr "Just beat the game" gushing.
Man what a satisfying ending.
Went loaded for bear: Frostner (Ice/Spirit), Draugr Bow/200 Ice Arrows, Padded Armor, all fully upgraded. Lox pie, blood pudding, fish wraps, all with spares. 10 Health Pots, 6 Fire resist. Base teleporter by boss. Cleared mobs in big radius. Raining. F***. Kill a day. Clear again. Sleep for 17 minute Comfort buff and full day.
Cut to: Comfort gone, night debuff, hammer at half durability. Naked dodge rolling past deathsquitos and Yagluth's huge meteors around shattered protective arches, desperately tossing rocks out of my bag so that I can actually loot my corpse, re-arm, re-buff, and dodge a mouth laser. Then, two furling spears out of nowhere.
The boss overall isn't that hard as-designed. Especially with portals. The key for me: the lingering blue flame AOE damage is irrelevant — stay mid-range to bait it out, then run in and smack him 3-6 times. Some of the fun was the craziness of going into the boss blind and dps'ing too slowly until things got out of hand.
But talk about emergent gameplay. I counted double digit furlings to deal with, the deathsquitos, a few poor lox. The sinking feeling of another hour or two grind for mats, because in endgame gear/buffs these boys still three-shot you.
If I'd lost, I probably say that a boss fight where random adds and day/night cycle becomes an issue is, if not a flaw, annoyingly unpredictable and RNG. As it stands, though, it was an immensely satisfying end to eighty hours of one of the best games that I have ever played. Can't wait for more.