The Aeseri twins, Odin and Loki, from "Devil Inside," are rampaging psychopaths released from the Alliance's maximum security containment unit to commit wanton havoc. Shooting down every Alliance operative that pursues them, Loki is captured and contents himself with playing sadistic mind games with Nick Logan, utilizing illusions and later attacking his way through the Alliance to commandeer one of their airships. Odin, meanwhile, makes his way to Iceland, attacking a submarine and a town in Iceland before making his way into the mountains. Once the two reunite, they reveal their ultimate plan: release and control the Frost Giant and cause an apocalyptic Ragnarok for no other reason than to thrill themselves, with Loki gleefully stating he can't wait until the two reach a populated city.
Lord Hanek is the vampiric leader of Intracom, a powerful family of vampires disguised as a worldwide organization. Hanek is behind several of Intracom's malevolent affairs, such as burning down vast sections of the rainforest to biting and turning several innocent people into vampoid slaves. Hanek demonstrates his vileness by attempting to have the Tree of Light destroyed, which would in the process completely wipe out the Banshee Sisterhood, and turns an innocent woman into his vampoid concubine to hurt one of the Alliance's agents. Hanek truly comes into his own in his last appearance in season 2's "Showdown": to power his teleporter technology, Hanek attempts to utilize the Banshee as living generators, in the process tortuously draining the life from them and eventually killing them. Hanek endangers the life of main character Nick Logan to lure Sh'lainn and the Sisterhood into his clutches, tricks the escaped Minotauri Kahn Mort into downing a helicopter filled with his own men into a hospital, and finally forces Nick into a Sadistic Choice involving the lives of Sh'lainn and his stepfather Nathan. Always possessed of a callous sophistication in whatever situation he's in, Hanek stands miles above the other major alien antagonists in terms of depravity.
The Shadoen agent Wraith is the one responsible for nearly all the misery within the story. Stationed in Earth back in 1946 to divide the alien races and allow the Shadoen to invade and conquer the planet, Wraith butchers the real James Rinaker and assumes his identity, bombing a Conduit safehouse and killing several innocent aliens to pin the blame on the Alliance's co-leader Walter Logan before trapping Walter himself in eternal stasis to take over the Alliance. As Rinaker, Wraith traps hundreds of innocent aliens in the Alliance's prison and has little compunction in arranging torture, bombings, and heavily destructive missions with little care as to who or what gets caught in the way, whilst simultaneously sabotaging several of the Alliance's missions and endangering several more lives. After his initial attempt to pin his crimes on Nick Logan, Wraith personally murders Ms. Smith-Heisen once she gets too close to the truth without an ounce of emotion on his face before discarding his disguise, rigging the entire Alliance bunker to explode and take out half of New Mexico, and savagely murdering Trueblood with his own claws. Rejoining the Shadoen fleet and still unsatisfied with how he's being treated, Wraith tricks the Shadoen high command into being nuked by the human army and spearheads the invasion himself, ultimately resolving to blow up the moon and kill off most of humanity to conquer what remains, with his only stated regret being the loss of "breeding stock" this will incur. A traitorous, manipulative sociopath even by the standards of the world-destroying Shadoen, Wraith is the most dangerous—and vile—alien to grace the series.
Narm: The oni are impossible to take seriously; unlike the threatening designs of the lycanthropes and the vampires, the oni have cartoony, exaggerated designs and silly designs that make them look more like caricatures of actual ogres (albeit ones more-or-less accurate to the depiction of oni in Japanese art), and any menace they could have had when one devours a Yakuza in its first appearance is quickly robbed from it when the next that appears is defeated by slipping on marbles, further compounded by the ridiculous image of it squatting down in a car it barely fits in.