Hmm...are you planning to describe him before or after he became a Daemon Prince?
If it's before, then I imagine him to be huge like all the Primarchs, but somewhat thin and more scholarly-looking than the others. Black hair, probably. He would be pretty fanatic and zealous, though, but that would probably show through in his eyes or expression ("his eyes burned with the fires of zealotry and fanaticism" etc). He would wear a combination of parchment-colored robes covered in religious text, and blood-red armor with silver trim (pretty standard for the Word Bearers...you could probably get away with a different style or color for the robes, but not for the armor). He might carry a massive book, too, or have an attendant carry it for him.
Afterwards, I'd say you have as much artistic license as you need to describe a Daemon Prince. Lorgar in daemon-form would be a lot bigger and more muscular than even a Primarch. The basic outfit would stay pretty much the same, but the robes would be tattered and stained and the text would have that warped Chaotic look, like maddened scribbling or blasphemous knowledge that would drive normal people completely insane to look upon. It might change and shift around a lot, too. The armor would be scarred from aeons of war, and it would be really mutated. Horns, spikes, sigils and symbols, mouths, fanged skulls, screaming faces, daemonic heads, etc, etc, etc. He might have antlers instead of horns on his head...nobody uses antlers very much anymore, but I think they're cool and unique. His eyes would glow or be fiery in whatever color you please, and probably wouldn't have any pupils or irises. They might even provide a glimpse straight into the chaos of the Warp. Clawed hands of course, and fangs or tusks, and he might have a long dripping tongue (maybe forked). He might even have a tail. If you're putting him in a building, he would probably sit on a throne in his temple-palace in the place where the god-figure would be, with an altar in front of/below him for his priests to make regular sacrifices to him.
I hope that's helpful...
arty:
