According to the Harry Potter Wikia:
While working at Borgin and Burkes, Tom befriended Hepzibah Smith, a
wealthy but elderly descendant of Helga Hufflepuff. Between 1955 and
1961, Hepzibah showed Tom her two most valuable treasures: his
ancestor's locket and her ancestor's cup. Tom's eyes flashed red with
greed and avarice upon seeing these objects and he desired them,
because he saw the locket to be rightfully his as Slytherin's heir and
the cup was a reminder of Hogwarts as well as a priceless artefact.
Tom killed Hepzibah to steal these coveted objects and vanished
without a trace.
Tom disappeared for ten long years. He slipped deeper into the Dark
Arts, travelled extensively, consorted with disreputable people, began
to become distorted in appearance and lost his handsomeness, due to
splitting his soul so many times, and began to use the alias "Lord