"It turns out there's an incantation far more potent than the one Pettigrew used to brew the Regeneration Potion back in fourth year. Don't look so surprised, I know everything there is to know about it. Anyway, the Dark Lord is convinced that thanks to this incantation he will regain his real, primordial body and all his magical and physical powers. That's why he hides so well. Because he feels he can't take any risk until the ritual is carried out."