"Silence, groundwalker", "What have I done? Created beauty by simply rocking! That's all", and "What is that sound? It's the pounding of creation's hammer on the anvil of time!" all reference Brütal Legend.
"Towerdiver, gotta get away, get away" references Holy Diver by Dio.
In life Karthus was a tally-man of Kindred and, to this day, he wears the vestments and carries the staff, which he still etches every time he 'releases' a mortal soul from life.
Unlike most Shadow Isles champions, who have a direct relation to the Ruined King and/or the Ruination that destroyed the Isles, Karthus is one of two champions with no connection to the Isles' origin (the other being Evelynn)