Marshals are Vanndar Stormpike's elite officers, most of whom reside in the Dun Baldar in Alterac Valley next to their general. At the start of the battle for Alterac Valley, Vanndar is protected by four Marshals, one for each of the Stormpike Bunkers; each is named for his Bunker, rather than given a personal name.
Tied to their general, Marshals cannot be pulled separately from Vanndar. The only way to deprive the Stormpike General of his protectors is to destroy the Bunkers before assaulting Dun Baldar. As each Bunker is destroyed, the associated Marshal is removed from the Valley.
Each Bunker must be protected carefully if the Expedition wishes to keep their Marshals, as there is no longer a way to gain more. Furthermore, each Marshal - and thus each Tower - represents 75 Reinforcements, a full 12.5% of the initial Stormpike army. Losing all four Marshals and Bunkers means losing half of the Expedition's Reinforcements. Still, losing Bunkers may be considered acceptable losses if other objectives are archived in the meantime.
Each Marshal is an elite of the level cap for the Valley - level 60 for 51~60 AV, level 70 for 61~70 AV. Level 60 Marshals have 61,000 health; and their level 70 counterparts have 140,000 health. Level 80 marshals have 327,500 health with no 25% more health buffs. Each generally requires his own tank and healer, so it is often advisable to destroy as many Bunkers as possible before beginning the encounter.
Patch 2.3 changes
Prior to 2.3, Marshals and their counterparts, the Frostwolf Warmasters, could be pulled seperately from their Generals. Additionally, the destruction of a Horde Tower would not only despawn a Warmaster, but also spawn an extra Marshal - and vice versa. Now, each side will only ever have the four guards they start the battle with.
Patch 2.4 changes
Vanndar stormpikes and balindas health have been reduced to match the total of galvangar and drek'thar. Warmasters/marshals now buff each other for 25% more health and damage. This is a stackable effect and is stacked seperately. so at 1 marshal the general has 25% more health and damage, 2 marshals 56%, 3 marshals 95%, 4 marshals 144%. This is compared to the generals base damage and health.