DISCLAIMER:Any theory that is posted is just in fun. I am posting these as a reader myself, imagining I'm in your shoes and what you would think. None of these theories have potential spoilers in them and can be taken how you want to take them. They may or may not be true, but that's up to you to decide.
You read that correct, is there a possibility Ulthor will return from the dead.
1-Dreams in the story appear prophetic, best cases are when Aeyron had dreams of Joanna wailing, ' “I had a dream……… Joanna was wailing in it…..…”'-page 566 which turned out to be Joanna giving birth to her children. Another scenario is when Gerold had a dream of Red men running into WinterFort, ' He saw WinterFort but it were up in flames, the ocean was inside the castle with hundreds of red men running into the castle and slaughtering the castle folk.'-page 199.
-Jon Darklyn had a dream while captured by Nestor Nik, it goes like:
'This thing was wearing full armour the colour of black onyx, they also wore a helmet that completely hid it’s face, the helmet was some sort of kabuto helmet they used in Takama-Go-Hara.
This dark lord even from a far was massive compared to Jon, looking like it were close to seven feet. His armour featured a series of buttons; in the middle of his torso were a collection of rectangle buttons encased in a square. Above this square was five grey vertical rectangle patterns and by his waist was a belt which wrapped around his body with even more buttons. To top it all of this dark lord had a cape the same colour as its armour.
In a quick sudden something appeared in its hand, it seemed to be a sword hilt but with no blade. Once again in a quick instance something appeared but nothing Jon Darklyn had ever seen before! A flicker of red appeared from the hilt with a great pish forming some sort of sabre. When the hilt moved so did the crimson light as if they were attached to each other, the sword was an elegant thing which gave off a low hum and illuminated the room in a red light.
The dark lord was walking closer to Jon now and was looking directly at him. From behind him Jon could hear another hum, one that was much calmer and more civilised. Jon tried to reach for his sword but saw he had none, in some sort of monotonic, nonnatural voice the dark lord spoke.
“I have been waiting for you O-“'
-We can identify the black onyx armoured thing to be Aeyron Archer because Aeyron has full black armour-'In his night black suit of armour the king looked like a demon.' and 'The spessartine gems flashed like orange flames in sunlight, the king wore no cape so it were the only piece of colour on his armour.'
-We also know that the suit of armour Aeyron wears is designed to make him taller, referencing 'the dark lord...was massive compared to Jon, looking like it were close to seven feet.', We know it increases
Aeyron's height due to this. 'even pushing his height past six foot, the frame slender and tall.'
-What further proves this is Aeyron is the colour of the sword he holds. 'A flicker of red appeared from the hilt with a great pish forming some sort of sabre',' the sword was an elegant thing which gave off a low hum and illuminated the room in a red light.' The sword is the colour of red and we know that Aeyron's sword is coloured red. 'Alongside the black steel there were little red ripples like veins across the sword, red tints which flickered and glimmered disappearing and reappearing as the sword moved.' Therefore we can conclude that the red sword Jon see's is Aeyron.
Now what does this have to do with Ulthor?
-'“I have been waiting for you O-“'' The dark lord says. Since this is Aeyron who would he been waiting for other than Ulthor? And the letter O sounds like the first two letters of Ulthor's name UL. This determines for me that Ulthor will come back and showdown with Aeyron.