I would say Azog, because he killed Thror, like Sauron's servants killed Isildur. But that's a name. And I could see it on a passport... "Azog of
Khazad-dum, ID#2542".
And I would say the Elven Smiths of Eregion, because I think it was they who forged the Seven, like Sauron forged the One. But they aren't an individual.
A name like "the Witch-king", or
Nazgul, seems to fit, except I see no connection to Thror.
EDIT: I think I have it! Dain I was Thror's father, and he was killed by a dragon, I believe.
Elendil was Isildur's father, and he was killed by...Sauron!
So the answer is...dragon!
Maybe... :P