N'Astirh made pacts with various villains in the Marvel Universe, and his first alliance was with Cameron Hodge. N'astirh's demons then kidnapped Artie Maddicks and Leech. Using Belasco's stolen book of spells, N'astirh sent demons to abduct mutant infants, and was brought computer expert Wiz Kid as a captive. N'astirh was informed by Cameron Hodge about Mister Sinister's orphanage for mutant children. N'astirh forced Wiz Kid to create a computer to process magic. N'astirh then transformed Hobgoblin (Macendale), giving him demonic features and powers. When Jason Phillip Macendale, then the Hobgoblin, offered to make a Faustian bargain to trade his soul for the power of a demon, N'astirh laughed and scoffed at the offer, thinking Macendale's soul to be too pitiful to be of any worth. In return for amusing him, however, N'astirh granted Macendale's request by fusing the hapless human with a demon, which would eventually break away to become the Demogoblin. Archangel vowed vengeance on N'astirh. N'astirh also made a pact with Madelyne Pryor, who used this alliance to get revenge against her creator Mister Sinister and her ex-husband Cyclops.
Madelyne Pryor, N'astirh and S'ym then plotted a demonic invasion, starting with Manhattan, known as the Inferno Crisis. Thanks to a pact made between N'astirh and Illyana Rasputin in the past, her actions in the present opened an inter-dimensional portal at the Empire State Building, through which N'astirh's demons invaded Manhattan. Kang the Conqueror sent his minion, the Growing Man to attack N'astirh's demon horde. During the invasion N'astirh also coerced Illyana and Meggan towards their darker sides—Magik having a tie to the demonic realm Limbo (transforming her into Darkchylde) and Meggan being an empathic shapeshifter who was swayed by the dark emotions around her (transforming her into the Goblin Princess). Illyana eventually closed the portal on her own, sealing the demons in Limbo.
N'astirh brought Madelyne Pryor to Mister Sinister's genetic lab at the orphanage, to rescue her baby, and to confront Mr. Sinister. Madelyne learned that she was merely a clone of Jean Grey, whom her husband had actually loved, and he left her upon learning that Jean was still alive. With the help of N'astirh and his fellow demon S'ym, Madelyne's psionic powers (far stronger than her progenitor's) were triggered, transforming her into the Goblin Queen. When N'astirh realized that Madelyne could not be used as his pawn openly he devised another way of coercing her to the dark side. When she was bound and defeated by Sinister he left her to fend for herself while he battled S'ym, and was infected and transformed by the transmode virus. The virus changes his coloring from green to red. He also claims he has 'become' magic, since he can now process and cast spells almost simultaneously.
N'astirh captured Nathan Christopher Summers. When N'astirh knew Madelyne had fallen into madness he returns and presents Madylyne her son as chief sacrifice.
N'astirh then attempted to form an alliance with Magneto. However N'astirh's invasion attempt was thwarted by the X-Factor, New Mutants, the X-Terminators and the X-Men, as Magik returned N'astirh's demons to Limbo.. He was frozen and shattered and ultimately destroyed by an overload of lightning driven into him by Storm.
Much later, N'astirh reappeared at the side of Illyana Rasputin, during the 'Quest for Magik' storyline, though the circumstances surrounding his return remain unclear. He does not, however, seem to possess the enhancements and abilities given to him as a result of his exposure to the techno-organic virus during "Inferno."
Vast magical powers grants him the ability to manipulate the forces of magic for a number of various effects, generate mystical energy as force bolts and protective shields, increase his size and strength, regenerate his injuries and tap into the innate evil of humans and turn them into demons with a touch. He also has the ability of flight due to his natural leather wings.
Transformation by the techno-organic transmode virus allows him the ability to alter his own shape and can rebuild his body from a single cell when destroyed or injured, and he can infect others with the transmode virus, converting them into "techno-organic" beings and absorbing their life energy. The virus also granted him a computer like mind allowing him to cast spells and process them almost simultaneously.
N'astirh has a gifted intellect, and possesses extensive knowledge of black magic gained through study of Belasco's principal book of sorcery.