Gain a Friendly reputation with the Gelkis, then speak with Uthek the Wise.
To beat the centaur, we must use their hatred against them. We will ally with one of the clans and through them, we will learn how to defeat all the clans.
If you choose to ally with the Gelkis then slay their most hated rival, the Magram. After you earn the respect of the Gelkis, meet with the Gelkis shaman Uthek the Wise, on the outskirts of the Gelkis Village, in southwest Desolace.