Found at http://www.wowpedia.org/Cairne
Also in the book The Shattering: Prelude to Cataclysm
Cairne, though impressed with Garrosh Hellscream's ability as a commander after visiting Warsong Hold in Northrend, opposes his selection as Thrall's replacement, and makes his feelings known to the outgoing Warchief; the old friends bitterly part ways as Thrall leaves for Outland, both leaders refusing to budge on their positions.
Upon receiving word from the owl Hamuul Runetotem sent, that the new Warchief sanctioned the killing of innocent druids (actually carried out by orcs loyal to Twilight's Hammer), Cairne issues a mak'gora, challenging Garrosh for leadership of the Horde. Garrosh accepts, but adds the caveat that the combat would not be the non-lethal combat sanctioned by Thrall, but by the old laws - to the death. Cairne agrees, and the combatants meet in the arena in Orgrimmar.
Each combatant is allowed one weapon and no armor. Cairne takes up his Bloodhoof Runespear, while Garrosh carries Gorehowl, the axe that belonged to his father, Grom Hellscream. The weapons are blessed by shaman; Gorehowl is blessed by Magatha Grimtotem, who seems approving of the new Warchief. At first gaining the upper hand, the Runespear is shattered by Gorehowl and Cairne takes a glancing slash in the chest. Cairne suddenly becomes faint and unable to raise what remained of his Runespear to defend himself, and he realizes that Magatha has poisoned Garrosh's weapon. Cairne's last thought is a lament - that he, who had lived with honor, would die betrayed, just as the howling axe of Hellscream slices into his neck. The tauren leader dies before he even hits the ground.
The moment Cairne died, Magatha orders the Grimtotem tribe to take over Thunder Bluff and the surrounding towns, murdering everyone who stood in their way. Baine, Cairne's son and heir, manages to escape, and with the aid of Garrosh and Gazlowe, leader of the goblins of Ratchet, retakes Thunder Bluff and exiles Magatha and the surviving Grimtotems. Baine is formally installed as leader of Thunder Bluff and chieftain of the tauren.
Cairne's remains are placed upon a pyre along with the shattered debris of his Runespear. Thrall returns from Outland to mourn him, regretting that he parted on bad terms with his friend. He weeps for the loss of a future lifetime of his friendship, his kindness, wisdom, and humor. Thrall speaks to the wind, hoping that Cairne's spirit hears him, to tell him that Cairne and the tauren were always the heart of the Horde - their true spiritual center, of forgiveness, compassion, and guidance. Thrall places his hand reverently on Cairne's now peaceful brow, to say a final goodbye to his old friend, and took with him the smallest piece of Cairne's Runespear to forever remember him and be in touch with his heart. The fragment was etched with a single rune, whose meaning did not escape Thrall - "healing".
Edited by Pxl on 11/24/2010 8:23 AM PST