What makes LV so powerful?

[Tandra]

No: HPFGUIDX 169015

This has been gnawing at me since I just finished the 6th book yet
again. I know LV hunts down power and finds out stuff that most other
wizards don't care to find out, but I don't get why he was so powerful
by the time DD met him in the orphanage. It seemed obvious to DD that
he wasn't an ordinary wizard, but where did all this power come from?
Because he was an heir of Slytherin? Is there anything that leads us
to believe that Slytherin was all that powerful? I just don't see why
he is all so powerful. Can anyone help me there?

TKJ