Lord Voldemort’s middle name in the Harry Potter books is Marvolo.
His birth name is Tom Marvolo Riddle. He is the son of the witch Merope Gaunt and the Muggle Tom Riddle. In Harry Potter and the Chamber of Secrets, the name is rearranged into the anagram “I am Lord Voldemort,” revealing how Tom Riddle adopts his feared identity.
Marvolo is also the name of Voldemort’s maternal grandfather, Marvolo Gaunt. The Gaunt family claims descent from Salazar Slytherin and preserves the Slytherin locket and the Gaunt ring, both of which later become important Horcruxes.
Morfin is Marvolo’s son and Voldemort’s uncle, while Merope is Voldemort’s mother. Those related names are common sources of confusion. Salazar Slytherin is an ancestor of the Gaunts, not Voldemort’s middle name.