Unnameable was a first shadow walker who took over a human at the dawn of humanity. With it, it gained intelligence and ambition to create something anew. It had unimaginable power and humans worshiped it, even carved effigies from the "ivory" it left behind, when it grew and shed its old skin. It however didn't want to be worshiped, it had a plan to remold earth with fire and stone, and then populate it in its own image. The ice-age defeated him, but it left pieces of himself behind inside the ivory carvings.
Andro Green found one such a carving, and he was possessed by the Unnameable, he then started seeking out the rest of the ivory pieces, and with each he found, he grew more powerful. He was stopped by Lara Croft who broke his horns, and then destroyed all collected ivory pieces but one, which she holds in her manor.