The leader of the opposition is Elder Crone Magatha Grimtotem. Magatha had been a mover and shaker for a long time in tauren society, due to her innate shamanistic powers. She took leadership of the Grimtotem by arranging marriage with the clan's leader, whereupon he died in an extraordinarily convenient "climbing accident." Since when do you see tauren climbing? The hooves seem like they'd be a large disadvantage.
Today, the Grimtotem are vocal in their opposition to the alliance of the tauren with the orcs and trolls. Bands of Grimtotem have conducted open raids on centaur, goblin and orcs villages on Kalimdor. Much like the night elf Archdruid, Magatha and the Grimtotem believe that their race is the only one with true claim to Kalimdor. Sadly, the Goon Squad has yet to figure out how to kite these two together so they can talk it out.
According to WoWWiki, elderly tauren are more likely than young ones to join the Grimtotem, and warriors and hunters are much more popular than shamans and druids. To join up, you need to demonstrate your ability to kill by bringing the head of a "lesser race" to a Grimtotem elder. Note: Do not try this with Grimtotem NPCs in-game, as they are unwilling to refrain from attacking you while you look for an orc head in your bags. You can only join as a tauren or a Forsaken, and if you get squeamish about killing, you're out. Not out of the organization -- out of this mortal plane.
So what are their nefarious plots?
Magatha wants to depose Cairne. No secret here -- she's been openly opposed to his rule for so long I'm surprised he hasn't drop-kicked her off the bluffs yet. Rumor has it that she'd like to kill him, but I see that as unlikely for a few reasons. Magatha is an elderly tauren, and Cairne is not only a force to be reckoned with in battle, he's extremely well-guarded. Even if she managed to knock him off through poison or such, the shock of losing their beloved leader would probably lead the tauren to choose a successor that follows in Cairne's footsteps.
The Grimtotem are attacking various settlements to try to drive other races away.
This is another easy guess, especially if you're Horde and have done the number of quests defending people from Grimtotems. (This includes the attacks on Horde settlements from Darkcloud Pinnacle in Thousand Needles and Grimtotem Post in Stonetalon Mountains, and the violence against Tabetha and Alliance settlers in the south of Dustwallow Marsh.) The "official" Grimtotem in Thunder Bluff deny a connection to these tribes, and Magatha has gone as far as to disown her nephew
for kidnapping and murder.
The Grimtotem like kidnapping stuff, and may be trying to forcibly steal females from other tribes.
The Grimtotem settlement in Feralas is based on kidnapping Sprite Darters for some reason. More disturbingly, the female tauren Kaya Flathoof
(in Grimtotem Post) and Lakota Windsong
(in Darkcloud Pinnacle) appear to have been kidnapped by the Grimtotem to become brides to the leaders and expand the tribe. Creepy, but not creepy enough to make you redo the escort quest in Darkcloud if you wipe on it.
The Grimtotems want to ally with the Forsaken. The "official" reason for this is that Magatha thinks the tauren can help the Forsaken get back to being human. Yeah, that's really believable. Can no one from the Horde take five minutes to go to Undercity and check out that excuse? The humans in cages seal the deal on that one.Anyway, several quests in-game suggest that Magatha wants the tauren to forge closer ties with the Forsaken for political purposes, whether to be her allies in an attempted coup of Cairne or to expand the Grimtotem's territory. One thing that's been hinted at that I think is a likely option is that Magatha knows the true purpose of the Royal Apothecary Society and wants to work with them to develop new poisons against her enemies. Ah, Magatha, why must you resort to such a stereotypical form of murder? Black widows are so passe. Go smash in someone's head with a hammer instead.
And in the comic book? The comic book reveals that Magatha is involved in a slave-using organization of Arena fighters. She has even offered some of her own warriors to an orc battlemaster so they can be "tested" in their fighting prowess, which doesn't sound great for the future of tauren stability. She may be contracting with human assassins to take out the mysterious fighter known as Lo'Gosh.
Appearance: Most Grimtotems are uniformly black-haired. They have a loose hierarchy and tend to refer to each other by class titles rather than by any rank. Aside from the usual tauren classes, Grimtotems can be mage- or rogue-like characters, so you can finally see that stealthed tauren you've been waiting for. It should be noted that the Grimtotem tribes often use shamanistic Spirit Guides to reach through the game and spread a poison of discontent on the Hunter forums.
For more information: WoWWiki on the Grimtotem, WoWWiki on Magatha,Oxhorn's Racing the Grimtotem.