the demiurg was basically described as a dwarf with incredibly bright hair. their technology is far advanced to the imperiums with grungni (fantasy players may recognise the name) being not a particularly skilled mechanic being able to rig up a teleport device out of pieces found on the starship which seems to be much more effective than imperial technology. also similar to the adeptus mechanicus he has extensive mechanical enhancement including at least a bionic eye and a bionic fist, which extends on a chain to be used as a mace in combat. in combat he wears no armour but seems to be incredibly tough and aggressive, taking down an ork warboss, one on one. doesn't seem to have any psychic capabilties but does seem to be an awesome soldier in combat, very tough, very strong because of the bionics and incredibly fast besides having the teleport technology.
if you re making stats i would imagine marine or beyond. basic demiurg i think would be a marine with a 6+ armour save for his prssure suit but a 5+ inv. save for cyber enhancement like guard can take.
also im not sure but i believe that demiurg rode trikes into battle when they were around. not certain about that might have been squats or jokaero you'll have to ask someone whos been playing longer.
checked on the nicassar they seem to be made of black oily smoke.
hope i helped with as little rambling as possible (failed that bit
