Phoresic Outerfolk

While it is not common knowledge, the Colossi and their servitors are not the only beings to have made their way across the Void.
Phoresic Outerfolk appear to have travelled alongside the Colossi, but to otherwise share no bond with them. Many such creatures travel as easily-overlooked eggs or spores, detaching from the Colossi when they sense a World to feed on. This category includes aboleths (travelling as miniscule eggs), ixitxachtli, otyughs and quori.
The Colossi are also accompanied by flumphs; diminutive, benevolent creatures that feed communally on mental energy, consuming excess psychic force with no ill effects to those they feed from. Although largely harmless, they have a vicious rivalry with the Beholders.
The gith are travellers in the Void who oppose the work of the Colossi, following the destruction of their own world. Travelling in the hollow carcass of a dead god, they pursue the migrations of the Colossi, seeking not to defend other worlds, but simply to destroy their ancient enemies. Gith culture is dedicated to the hunt - to pursuing and destroying the Colossi and their servants, in particular the mind players.
Although not native to Aious, the gith are considered - by the few who know of them - to be effectively a mortal lineage. They are not thought capable of producing offspring with the mortals of Aious, but half-gith children with other lineages from their own, destroyed world are known.
The gith are divided between the githyanki and the githzerai. The githyanki are hardline conservatives, devoted to the hunt and the destruction of the Colossi at any cost. Socially, they are the dominant sect, and often persecute the githzerai to maintain the focus on their cause. The upper echelons of gith society - the arcanist nobility and the martial gentry - belong to this philosophy. The githzerai hew instead to an aesetic philosophy, led by a cadre of warrior-mystics, and broadly favour the protection of other worlds from the Fate of their own.
Although vanishingly few and far between, other mortals have travelled to Aiaos across the void, including strange Elves, the simian technocrats of the hadozee, and the militaristic giff. None of those who have arrived out of the Astra have ever been able to leave again, and small numbers of these alien lines - most notably the giff enclave known as the 3rd Arquebussiers - live quietly among similar populations.