Warmasters are Drek'Thar's elite officers, most of whom reside in the Frostwolf Keep in Alterac Valley next to their general. At the start of the battle for Alterac Valley, Drek'Thar is protected by four Warmasters, one for each of the Horde Towers; each is named for his Tower, rather than given a personal name. The exceptions to these are Warmaster Laggrond and Warmaster Garrick.
Tied to their general, Warmasters cannot be pulled seperately from Drek'Thar. The only way to deprive the Frostwolf General of his protectors is to destroy the Towers before assaulting Frostwolf Keep. As each Tower is destroyed, the associated Warmaster is removed from the Valley.
Each Tower must be protected carefully if the Frostwolf wish to keep their Warmasters, as there is no way to gain more. Furthermore, each Warmaster - and thus each Tower - represents 75 
[35-60] Reinforcements, a full 12.5% of the initial Frostwolf army. Losing all four Warmasters and Towers means losing half of the Frostwolf Reinforcements. Still, losing Towers may be considered acceptable losses if other objectives are archived in the meantime.
Each Warmaster is an elite of the level cap for the Valley - level 60 for 51~60 AV, level 70 for 61~70 AV - and uses 
[Charge], 
[Cleave] and 
[Whirlwind] attacks. Level 60 Warmasters vary greatly on health, ranging from 27,000 to 60,000; but the level 70 versions all have 140,000 health. Each generally requires his own tank and healer, so it is often advisable to destroy as many Towers as possible before beginning the encounter.
Patch 2.3 changes
Prior to 2.3, Warmasters and their counterparts, the Stormpike Marshals, could be pulled seperately from their Generals. Additionally, the destruction of an Alliance Bunker would not only despawn a Marshal, but also spawn an extra Warmaster - and vice versa. Now, each side will only ever have the four guards they start the battle with.