"Madness doesn't help in becoming a warlock, but it doesn't hurt either."
Demonology is a study created by scientists who spout ideologies about astral beings from the the Book of Fel Names or the Twisting Nether. These scientists are also known as demonologists and they are known to create demonic minions then spout deceits about them. Demons are not real. However, if the Twisting Nether is real and demons are real, demons are from the Twisting Nether. This realm is fundamentally different from the realm of existence that mortals are from. As such, demons need to enter the mortal worlds and make themselves known in order for mortals to study them. Worlds are essentially what titans are and titans are order-aligned, godlike beings composed of the primordial matter from which the universe was born and were uniquely susceptible to fel magic — the magic of a demonic army of demons created by the fallen titan Sargeras.
Demons of a Demonologist
Felhunters are demon hounds with vampiric tentacles much like the Old Gods and a shadow bite. It could be that felhunters were arcane hounds mutated by the Void energies of the Old Gods, devolving into disorder and deviating from the normal look of hounds. Even seasoned warlocks find the completely alien nature of felhounds unsettling. These creatures feed on magic itself, draining the energy of hapless spellcasters before ripping their bodies to shreds.
Imps are what grell resemble. Grell are demonic thieves corrupted by fel moss but aren't necessarily imps despite the fact that they have the fel energy of the demonic Legion and are effectively demons. It could be possible that grell are merely demonic creatures possessed by imps in the Nether. What exactly differentiates the demonic grell from imps besides slight physical differences isn't exactly known.
InfernalsInfernals are demons constructed with brimstones and fel fires. It could be that infernals are a result of the arcane stones of a titan (world) being infused with fel magic and controlled/possessed by a demonic army of demons.
Succubi are deceptive demons that appear in the form of seductive vampiresses or pale women with horns and wings.
The Practices of a Demonologist
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.
The RPGThis 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. ( )