My lore for him is that he is a rogue Makuta who got mutated. He created the Skull Spiders as a subspecies of Visorak. He bound himself to the Heart of the Visorak and took over the Visorak horde after Vakama freed them. The lord would however die when the Toa Mahri wiped out the Visorak.
Nice, the head seems perfect for him and the abdomen is nice part usage. My crit would be that the legs do not seen like they could hold his weight, on a real spider they often have larger legs at the front and I think here it would work well as he has a lot extra weight near the front plus lengthening the legs a bit might be good.
I think i like this even better than the Umarak one! The use of a tridax torso as an abdomen (as Axelford pointed out) is really good. That skull Fikou thing also looks really good.