Herobrine, the Unseeing Wizard, is the leader of a vast army made up of different species of monsters, from the undead in all forms to treacherous villagers such as Urf.
Herobrine was one of the major combatants in the First and Second Great Wars. He fought against the kingdoms of Aetheria and the Twelve Heroes in the First and Second Great War. The remnants of his devastation can be seen in the Undead and other Monsters that plague Aetheria to this day. While he is powerful, he is not a god, and therefore Divine Law does not affect him.
Herobrine's motives are unknown, though some speculate it could be world domination or termination. One of his most notable appearances was when he appeared in one of Runt's dreams, where he claimed he was coming for him and his village. Since Notch can bring Terrarians to Aetheria, his ultimate goal may be to travel to (and take over) Earth.
He was defeated in both the Two Great Wars and when he was defeated in the Second Great War, he had a protective spell put on him which whisked his spirit into a "phylactery", otherwise known as a soul-crystal. The crystal was discovered by the Knights of Aetheria, who guarded the crystal. Herobrine's servants, scattered around Aetheria, came together and defeated the Knights of Aetheria, reviving Herobrine. His homeworld is Icerahn, the 14th dimension, otherwise known as Icehollow.
Herobrine was weakened from being revived, once level 255, but his power is still rivaled only by Notch. He is fast and strong and has the ability to fly. He has mostly spell abilities, including a lot of supportive stuff- buffs, gate-type abilities that allow a small army to travel pretty far, etc.
Herobrine is able to create monsters for his own personal gain, namely the Undead and possibly Creepers. It had been said by Breeze and Brio that they had been locked in a dungeon and experimenting on.
Herobrine is able to utilize the power of mobs to his advantage, empowering them and commanding them. He is able to make monsters spawn at night. Villagetown is stated by Runt to merely be a testing ground. When the monsters are able to destroy Villagetown successfully, he will attack the larger villages to the west.
His main army may be more powerful than any that has been seen previously, with magically-enhanced monsters wielding weapons of terrifying power.
He is also in alliance with Enderstar, an exiled enderman who currently controls a great fraction of the Nether's forces. This means that Herobrine controls the Nether mobs as well as the Undead, even though Enderstar might be able to break off.