Zmodlor

Zmodlor is a minor demon who specializes in infiltration. During the events in Cycle of Hatred he recruits citizens of Orgrimmar and Theramore into the Burning Blade and uses them to raise tensions between the orcs and humans. He was banished thanks to the combined efforts of Aegwynn and Jaina Proudmoore.

Zmodlor and Aegwynn
Zmodlor once took over an entire schoolhouse full of children in the village of Jortas. Zmodlor was preparing to perform a ritual that would have poisoned the souls of the children when the Guardian Aegwynn discovered his activities and banished him back to the Twisting Nether.

The Third War
Eight centuries later, Zmodlor was revived by the Burning Legion to supplement their second invasion. He survived the events at Mount Hyjal and began instigating war between the orcs in Orgrimmar and the humans in Theramore.

Cycle of Hatred
Zmodlor recruited both orcs and humans who were prejudiced against one another into a new incarnation of the Burning Blade cult. As an incentive, he promised to help his new followers destroy their enemies whether they were humans who hated orcs or vice versa. No one who followed him was aware that he also recruited their enemies. Among the members of Zmodlor's Burning Blade were advisors to both Jaina Proudmoore and Thrall: Kristoff, Jaina's chamberlain, and Burx, one of Thrall's chief warriors.

Zmodlor's plan would have succeeded, but for Jaina and Aegwynn's timely intervention. Meanwhile, Jaina and Aegwynn confronted Zmodlor and several of his worshippers atop Dreadmist Peak. Zmodlor was banished by Aegwynn's spell, cast by Jaina Proudmoore, moments before his plan would have succeeded.

While speaking with Thrall following these events, Jaina states that the Burning Legion would have a hard time rescuing Zmodlor a second time and that because he is a minor demon it would not be worth the effort. For the same reason, Aegwynn did not believe that the Burning Blade's activities were the result of Zmodlor acting on his own. Aegwynn believed that "a plan of this complexity is far beyond him". The identity of Zmodlor's master remained a mystery.

Zmodlor's Master
Though it was never hinted beyond Zmodlor's master being smarter, it is reasonable to assume that the Shadow Council in Felwood was ultimately responsible for the plan because the Burning Blade is part of the Shadow Council.

Zmodlor's Race
Cycles of Hatred never specifically states what kind of demon Zmodlor is, but he is briefly described as a huge demon with leathery skin, bat wings, and flaming eyes.

There are only three types of demons that fit any of the description: Nathrezim (Dreadlord), Doomguard, and Annihilan (Pit Lord).

All three types are known to have bat wings and appear to have leathery skin. All three types could be considered "huge" by a human with Annihilan being the largest by far (Nathrezim are the shortest at over 8 foot tall). However, only Annihilan have been known to have flaming eyes.

Considering all of this, Annihilan fits the description best, but the events also take place in a cave on Dreadmist Peak. We do not know how large the cave is supposed to be, but it casts doubt on the idea of fitting a large demon inside the cave.

On the other hand, Zmodlor's work involved a lot of subterfuge, which is generally considered to be the specialty of the nathrezim. The Annihilan were never portrayed as particularly intelligent or cunning, relying mostly on brute force.

More information on demonic traits is listed on the demon article.