The subject of this article or section originates from non-canonical sources. To find out about what is considered "canon" see LOTR:Canon.
He is not from the works of J.R.R. Tolkien.
Zog was a skilled Orc mystic with a devout faith to Sauron. As required of his master's command, he and his group of acolytes went to Gorgoroth to use their sorcery to call Tar Goroth to take up arms in Sauron's servince. However, the demon proved too much of a challenge for even Zog to tame, shown true by how the Balrog turned upon the acolytes before fleeing to the surface.
He remained a genuine servant to Sauron still, raising undead Orcs and slumbering followers of Morgoth back to life to threaten the plans of the ascending Bright Lord. However, his success only increased his ego and thirst for power, such that he forsook his allegiance to Sauron. In his lust for mastery, Zog committed himself to raising an army to contest his master and rule Mordor. To achieve his purposes, Zog spread his web of allies and agents throughout the Black Land in the hopes of seizing the right opportunity to strike.
Upon Tar Goroth's fall beneath the icy surface of Seregost, Zog and his acolytes went to the frozen lake in an attempt to summon the Balrog and dominate it. The Bright Lord, however, intervened and stopped the ritual by crumbling the totems and seemingly slaying the Uruk Necromancer.
In Minas Morgul, Zog was resurrected by his loyal followers. Intent on supplanting Sauron, the necromancer began to replenish his depleted army by reanimating the fallen Orc soldiers who had claimed the fortress in the first place. Once more, the Bright Lord arrived to challenge him and was able to overpower and defeat him. Carnán, in the form of a Drake, flew over and burned his remains so that he could never return. However, as Zog often reminded his adversaries, a true necromancer was said to never stay dead for long.
Zog is also the name of the Main antagonist from a Frank Baum Novel