“Look Ayden, if or when Kaia comes for you. You just need to know that he'll offer you something you really really want. But, and this is important, there's a price for it.
"The price now, looking at it right now, is so much worse than it seemed when I was talking with him. After the fright and the dread and the intimidation, the deal I got was NOT worth it.”
- Seline, Bomoki's Gate
Kaia, "The Halfling of Legend"
Kaia is not a halfling. You may have felt this from the books. He is an exiled Lord of Hell, the 8th actually. When Mephisto took the 8th and then conquered the 7th, Kaia left to see what ways might present themselves to gain enough power to recapture the 8th. He knew Warp was fake. He knew that without a throneplane, he had no chance especially against a hell lord with now 2 throneplanes.
He found his way to Tehra when Tiamat conquered the 1st plane of Hell. He helped Her to do so, curious to see if an outsider could succeed in taking what Warp created. He noted the burning wrath of Takhissis and the searing rage of Alerius. When the dust settled, he said, "Hi."
Alerius brought Kaia to the Forsaken Isles long before there was ever an Allegiance of Blood. Kaia is allied with Tiamat to the point that he will not seek converts from Tanians. Additionally, without a throneplane, he has no divine power to build faithful worshippers. Instead, he looks for up and coming heroes and bargains with them against some future owed favor. Studying his bargains, he then arranges people, monsters, and other gods into bets. His ultimate purpose is to curry favor with the gods themselves, of any pantheon.
He doesn't look this way either. He's black skinned and has vampire fangs. Like the Court of Patriarchs wandering the world as humans, Kaia learned that his shapeshifting could not entirely hide his hell lord nature. The 8th is home to the eldar fiends named "Varless." They would look a lot like the balrogs in LOTR. Kaia is their racial god. If he can gain enough influence to break Mephisto's hold on their minds, he could re-ascend. He doubts it though and is playing a longer game... one that involves Tiamat, Asmodei, and other gods with the sole purpose of killing Mephisto.
Mephisto thinks Kaia died when Tiamat took the 1st plane of Hell. Alerius protects him in Tania and provided he stays within Tania's borders, Mephisto and servants of Mephisto cannot detect that Kaia is the fallen lord of the Varless. The Halfling of Legend though has a certain charm and many heroes in other nations pilgrimage to Tania hoping that they will be deemed worthy enough for Kaia's strange visits.
Kaia deals with Tanians are documented in the Temple. They include:
- Dar Tania - a lock of her fiery hair to Kaia to deliver a message to Pha Rann for Sean in Heaven. It is not known what the message was though sages speculate it was hope that Sean join her in Tiamat's afterlife
- Dar Ana - Her mortality for ability to vampirically take and redistribute life. This made her the first necromantic battle priestess; see Necromantic vs Augmentor Clerics
- Daryx - his drow noble powers for the ability to survive Lolth's kiss
- Rojo - location of conversion-possible dragons for a future IOU; which was to not ascend to Tiamat after 1st Immortality Rite, but to complete all Ascension Rites and then go to Tiamat as a peer
- Malcor - for a future IOU, Malcor requested wisdom to understand the River
- Calvin - for a future IOU, Calvin asked for guaranteed acceptance into a paladin order and got a nice shield
- Seline - for a lifetime's vow of total chastity (to never know a male's touch), Seline received mastery of all bladed weapons
If the dragons themselves made bargains with Kaia, that is not recorded.