Baby Connor is taken from Angel by Wesley Wyndam-Pryce when Wesley's throat is slit he is left to die by Justine Cooper. Justine, intent on raising the child with Angel's nemesis, Holtz, meet Holtz near a bridge and soon find themselves in a cross fire between Wolfram and Hart, Angel, and Sahjhan. Using this moment of opportunity, Sahjhan opens a dimensional rift into the dimension of Quor'Toth, calling it the "darkest of dark worlds". Holtz takes the baby and runs into the dimension with Connor in arms. The rift closes up, leaving Angel devastated.
Days pass, and as Angel determinedly tries to find a way into Quor'Toth, Lorne speaks to his contacts. He claims "very few have heard of it, and those who know of it are frightened by the mention of its mere name." He further explains that in order to open a dimensional rift into Quor'Toth requires very dark magic. Magic that would come with major consequences. Angel doesn't listen and continues to seek a way into Quor'Toth.
He attempts to create a portal but soond discovers that no portals can be created, only rifts: tears in reality itself, which are too dangerous and powerful for Angel to be able to create. He performs a dark ritual to get the dimensional demon Sahjian to create another rift, but is informed that Sahjian only had the power to create the one rift--the one that sent Holtz and Connor in. A pentagram of blood remains in the lobby of the Hyperion from this ritual. Later, after a day or two, a man comes in to Angel Investigations, looking to see if they can help him. Upon leaving, thinking no one is there, a clear slug-like creature forces itself down his throat and begins to drain him of fluids to the point that he turns to dust.
After discovering more of these things at the bottom of the Hyperion Hotel, Angel and the others try to rid the place of the intruders. With the help of Cordelia's demon powers, they eventually manage to rid the place of the monsters, but before this Fred is infected and the slug inside her speaks of a "Destroyer" that is coming to kill Angel. (Fred is cured when Gunn forces her to drink alcohol, which dehydrates the body and makes the slug want to leave.) Soon after ridding the place of these creatures, a large creature comes through a dimensional rift from Quor'Toth and prepares to attack until a mysterious person comes through and quickly kills that creature effortlessly. The man who comes through is none other than Connor, who has already aged drastically within a matter of days upon being in Quor'Toth, revealing that time passes much more quickly in that dimension.
Other that what is known, a person who is raised in Quor'Toth can become slightly, if not drastically, darker while living there. Cordelia's demon powers cleanse Connor's soul, and she reveals that it was made harder from living in Quor'Toth. It can be assumed that it would probably be populated mainly (if not entirely) by demons.