Ciel is a sickly/dying human that TV Sebastian decided to take in on a whim, when Sebastian found him after Ciel's family was massacred by a pureblood.
I'm just going to be lame and repeat what Andrea said above: they're visitors looking to a) track down the pureblood who killed Ciel's family you killed my father, prepare to dieee!1 style, b) find a pureblood who's willing to turn Ciel and prevent his death, and c) find a way to bind Sebastian to him permanently through blood-bonding. Because he likes his interdependent relationships and that's how he rolls.
So yes, definitely looking for a few pureblood volunteers.