The earth speaks to me. And it says you have killed many Magram. This is good. Leave! The Magram do not speak with the weak. We kill them!
There are several centaur tribes still vying for dominance in the wastelands of Desolace. Both the Horde and the Alliance have their own reasons for being wary of the centaurs (tauren especially), but are not above playing them off against each other. Two of these clans are the Gelkis and the Magram.
The Gelkis live in southwest Desolace, just west of the road to Feralas. They are led by Khan Shaka and his shaman Uthek the Wise. The Magram dwell in southeast Desolace, east of Mannoroc Coven and south of the lands of the Kolkar centaurs. They are led by Khan Jehn and his spokesman Warug. Gelkis mobs range from 28-37, Magram mobs from 29-38.
You begin at 2000/3000 Neutral with both sides. When you reach Friendly with the Magram, you can talk to Warug and begin the Magram questline. When you reach Friendly with the Gelkis, you can talk to Uthek the Wise and begin the Gelkis questline. There are no other benefits from gaining reputation with either side.
Killing mobs of one side earns 20 rep with the other side and -100 rep with the faction you're killing as long as the mob is green or better to you. (Some report that killing the named mobs earns 25/-125 rep) Killing grey mobs yields significantly less faction gain, but continues to lose the full 100 rep with the attacked faction.
Both the Horde and the Alliance are interested in these tribes - the Horde would like to weaken the centaurs, the Alliance wants to know what weaknesses they have.
Gurda Wildmane, Ghost Walker Post Gain Friendly reputation with the Magram, then speak with Warug 2100 xp, Chains to "Assault on the Kolkar"
Kill Gelkis centaurs to become Friendly with the Magram. Warug is located up the hill in the mountains just northeast of the village, near (75.0,68.1).
 Assault on the Kolkar Warug Kill 12 Kolkar Centaurs, 12 Kolkar Scouts, 6 Kolkar Maulers 2650 xp, Chains to "Broken Tears"
 Broken Tears Warug Destroy three Tears of Theradras in the cave (Bolgan's Hole, up the ramp on the southern side of the excavation) in the Gelkis Village and retrieve the broken pieces 2650 xp, Chains to "Gizmo for Warug"
 Gizmo for Warug Warug Bring Warug an Advanced Target Dummy 2100 xp, Chains to "Khan Shaka"
Advanced Target Dummies are crafted by Engineers with a minimum of 185 skill. You can usually find them on the AH for 25s-1g.
 Khan Shaka Warug Slay Khan Shaka of the Gelkis and return his head to Warug 2850 xp, Chains to "Khan Hratha"
Khan Shaka is a L37 non-elite and is usually accompanied by 2 guards (L35-36). He is in the southeastern corner of Bolgan's Hole (the cave where the Tears of Theradras were found for the "Broken Tears" quest), at the bottom of the cave, near (40.5, 95.5).
Gurda Wildmane, Ghost Walker Post Gain Friendly reputation with the Gelkis, then speak with Uthek the Wise 2100 xp, Chains to "Raid on the Kolkar"
Kill Magram centaurs to become Friendly with the Gelkis. Uthek is located near (36.3, 79.2) in front of a tent at the north end of the village, just south of the road to Shadowprey Village and north of the excavation.
The bags are usually on their own around the tents; you can identify them by mousing over them and looking for the gear cursor.
 Ongeku Uthek the Wise Kill Ongeku and retrieve a Draenethyst Shard 2850 xp, Chains to "Khan Jehn"
Ongeku is a L37 non-elite who spawns (randomly) near one of the huts at the eastern side of the Fallow Sanctuary area of the Swamp of Sorrows. If you follow the northern road through the Swamps, you will eventually find the sanctuary. The area is populated by (hostile) L36-39 Lost Ones (degenerate Draenei) and patrolling Horde guards (which aren't a problem if you happen to be Horde).
 Khan Jehn Uthek the Wise Slay Khan Jehn of the Magram and return his head to Uthek. 2850 xp, Chains to "Khan Hratha"
Jehn usually spawns in a big tent with banners out front at the southwest end of the Magram Village, just east of the northeast entrance to the Valley of Bones (near 66.0, 80.5), accompanied by a L36 non-elite guard.
This faction exists primarily to introduce characters to grinding for reputation. There are no significant bonuses to gaining faction with them other than the opportunity to complete the questlines. From the mob and quest levels, it appears that the questline encourages characters to complete the Magram quests, then turn around and grind back to Friendly with the Gelkis and complete those quests. Look at the rewards and decide which items you want to end up with to decide if this is worth it. Rogues and Druids may wish to consider allying with the Magram for the belt; the Gelkis otherwise seem to have superior items, although Uthek forces you to take a long detour off to the Swamp of Sorrows.
You need 1000 points to become Friendly with the initial faction. This will require 50 level-appropriate kills (earning 20 rep points each). This will cost you 5000 points with the other faction, leaving you at 0/3000 Unfriendly (at the border to Hostile). To grind back to Friendly with the other faction will require 6000 points or 300 level-appropriate kills. Note that the initial quests are L33-35, with the end of the chain L42. In general, this will not be a profitable quest chain if the majority of mobs are grey (L45 or higher).
The quest " Centaur Bounty" given by Corporal Melkins at Nijel's Point (Alliance) or Felgur Twocuts at Ghost Walker Post (Horde) has you kill centaurs for their ears. If you haven't already completed it, make sure to grab this quest as well, since you'll be killing a lot of centaurs.
"[42+] Khan Hratha" is the shared final quest for both questlines. You must first obtain the mouthpiece for the horn by killing Maraudine centaurs until the mouthpiece drops. These are found in the Valley of Spears in western Desolace (northeast of Shadowprey Village, due west from the Kodo Graveyard). This is the entryway to the Maraudon instance. You then climb to the top of the stone tower (north of the entrance to Maraudon, near (29.8, 53,3)) and use the mouthpiece on the Maraudine War Horn. This summons three waves of Maraudine, each consisting of 2 melee and 1 caster centaur, all L40 non-elites. There is sufficient time between each wave for drinking/eating. The casters can heal, so they should be taken out or otherwise neutralized first. Khan Hratha accompanies the third wave and will despawn in approximately 5 minutes if you don't kill him before then. Immobilize him and take out his guards first; if possible, pull the guards some distance away to give you more time. These mobs also have a chance to drop another mouthpiece, which you might want to save if you're going to try doing the other questline.