My friend got dark souls from his friend, and he entered the painted world. He is on Vanilla and is playing offline, and somehow, Black phantom infected undead started showing up. He asked me why they were there, and I was puzzled. Here is how he stated they started showing up.
"Enter painted world hollow, die a couple times, go human, see black phantom enemies, die, stay hollow, but still see them, die again, and they vanish."
He doesn't know how he triggered them, and by the time he texted me, they were already gone. Hes gonna try and spawn them again so he can hopefully track down the summoning sign, but either way, this might prove that the "Gravelord Servant" Boss exists. To the painted world I go.
EDIT: He was also in the way of white.