Demonology is presumably a branch of theology (or study) relating to a demon (or to demons).
By their very nature, demons are leeches on the living universe—but the demonologist has mastered harnessing the power of these malefic beings on the field of battle. Warlocks harvest the souls of their defeated enemies; those specialized in the ways of demonology use this life essence to tap into the Void, pulling all manner of abomination from the chaos of the Twisting Nether. While such a practice is often considered by outsiders to be wicked and reckless, the demonologist maintains absolute control over the summoned creatures. These malignant entities are fully beholden to—and empowered by—the will of the warlock, until banished to the realm from whence they came.
Demons would be non-physical entities (incorporeal spirits—which are undead) because demons are from the Twisting Nether, an astral dimension separate from the physical universe. The physical is separate from the non-physical and if demons are physical beings, they would not be able to exist in a non-physical place. However, despite knowing that demons can't be physical beings who exist in a place separate from the physical universe, Matt Burns—Writer at Blizzard Entertainment—stated that he thinks there is physical stuff in TN when he was asked if demons are non-physical beings that warlocks can give physical shells to. He presumably stated this because warlocks would be necromancers.
Because demons are from the Twisting Nether, demonic power (or warlock magics) would be an astral power (or a necromantic power) that exists in - and is derived from - the Twisting Nether. However, when asked if perhaps warlocks were harnessing their power from the Twisting Nether, he replied, “No, that’d be magi, IIRC. They gain power by the destruction of another source (recall the undead warlock in original cinematic) Destruction is the process of destroying constructs, breaking them down into the parts that they are constructed of. If warlocks get their power from destruction, they would get it from the destruction of a constructed, demonic source because their power is demonic. Demonic power cannot be demonic without relating to a demon and if demons are from the Nether and are not the source of demonic power (something they contain), the source of demonic power would be the place where demons are from, which is the Twisting Nether. It is known that a warlock can twist a soul to power destruction and in an ancient language, there was a single word for "twisted souls". These were the things that were held in check by the warlocks' will, but just barely. Twisted souls. Dae'mons. Demons. Held in the thrall of unbridled hate and malice, they hungered for nothing less than the destruction of all life in the universe. So by drawing souls and twisting them, warlocks seem to be converting life essence into the chaotic, demonic existence of the Nether and controlling those twisted souls.
In battle, warlocks who command the power of destruction favor incantations of pure chaos and aggression in battle. In this regard, they’d find a stronger kinship with fire mages than warlocks of other disciplines—if not for their propensity to make use of magic deemed detestable by all mage orders. The destruction warlock is well-versed in discharging a dizzying array of shadow, fel, fire, and chaos magics upon opponents that rattle souls and conflagrate bodies. They require little motivation for the havoc they wreak, happy to revel in the destruction they cause—thrilled at any opportunity to watch the world erupt in discord around them.
In the RPG EditThis section concerns content exclusive to the Warcraft RPG, and thus unlikely to be canon.
Demonology is the study and practice of using and amplifying demonic powers and the ability to summon items or demons, mostly using soul shards. The craft of conjuration is the primary focus of the warlock, and in taking on the mantle of a demonologist the warlock sacrifices knowledge in other areas of arcane practice. ( )