Lord VOLDE-MUTT is one mean evil dawg. He is responsible for the Murder of Hairy Potter's Parents when Hairy was but a pup.
His attempt to destroy the young Hairy resulted in him being destroyed himself, or so it was believed. The scar on Hairy's head was the only sign left after Volde-mutt was destroyed. This scar would link Hairy to Volde-mutt and vice versa.
Lord Volde-Mutt spend many years as a formless spirit, gradually starting to reform himself, with the help of other evil magical mutts, however the meddling Hairy Potter and his mates stopped Volde-Mutt from becoming fully reformed and re animated on earth many times.
BUT He who cannot be named waits for his chance to return and wreck havoc on the world, and on that return he will have to destroy Hairy Potter before Hairy can destroy HIM.