Gain a Friendly reputation with the Gelkis, then speak with Uthek the Wise.
We are fortunate the centaur clans fight among themselves, for if they joined into a single force then they would be unstoppable. Yes, we are fortunate, and we should press our advantage.
One of the centaur clans, the Gelkis clan, is in brutal war with the Magram clan. If we attack the Magram, then perhaps an alliance may be struck with the Gelkis.
Speak with Uthek the Wise, in the outskirts of the Gelkis camps in southwestern Desolace. Through her we might strike an alliance with the Gelkis.