Wanda Maximoff (Scarlet Witch)Edit
Wanda's main power is the superhuman ability to manipulate chaos magic, granted to her by the High Evolutionary's experimentations and further amplified by the demon Chthon when she was born. Her hexes were, by extension, simple, indirect manifestations of this magic, destabilizing probabilities by affecting energy fields and matter, ultimately inducing chaos. When she acted as a conduit for the planet's life force, her powers were further amplified and not always under her conscious control, and their effects persisted even after Dr. Strange's Eye of Agamotto shut down her mind. Even the combined efforts of the Sorcerer Supreme and Earth's most formidable psychic mind were unable to fully inhibit such effects for long.
While Wanda is stable and healthy, she wields her powers with proficiency and is capable of triggering and controlling multiple occurrences at once; one of her most famous feats was reviving Wonder Man seemingly from another plane of existence. While she served as a conduit for the Earth's life force, her power was so great that she was able to generate armies from nowhere and has also altered the entire Earth-616 reality at will and permanently shifted the nature of all possible futures without realizing what she had done. Wanda was also shown to easily wipe any in her own illusory reality from existence with but a thought. In the House of M reality that she created, her chaos wave was so powerful than even Meggan Puceanu, who absorbed the power of beyonders, only could stop it for a few seconds. Wanda even casted one spell that caused the loss of most of the mutant population through the deletion of the Mutant Genome created by the near-omnipotent Celestials on a multiversal scale. It was revealed by Doctor Strange to Beast that this lasting spell was of such complexity and strength that it had woven itself into reality and the world of magic itself and that if he were to even try to reverse it, all of reality would implode. Wanda was even able to heal the severely damaged legs of Charles Xavier while all his Earth and alien friends, allies and colleagues' efforts apparently failed for countless years.
She is actually considered the very "nexus-being" of the central universe of Earth-616, serving as the entity that personifies the realm's character and serves as the focal point or anchor of that reality. This makes her along the scales of class and power as other known nexus-beings, such as Merlin, Sise-Neg, Kang the Conqueror and Leonard Tippit.
The nature of Wanda's ability to channel and wield magical energy has been altered many times by different writers. It was initially brought about by the demonic sorcerer Chthon. One the day that Wanda was born, the ghostly specter of Chthon visited the newborn Wanda and "blessed" her so that she could channel the dangerous chaos magic that was the basis for Chthon's unholy might. Chthon's intentions were to create a vessel that he could possess one day, when Wanda was a full-grown adult, though his schemes to do so were foiled by the Avengers.
As such, due to Chthon's "blessing", Wanda is also vulnerable to having all of her powers disabled if Chthon is ever disrupted. During the period in which pages from Chthon's sacred tome, the Darkhold, were scattered across the globe, Wanda's powers were periodically disabled -- with her being left powerless in several dangerous battles against various super villains -- until several of the missing pages were restored to the book. Although this particular plot thread was stated by Agatha Harkness who was reanimated by Wanda. How much of it is true and how much of it is more unconscious manifestations of Wanda's reality manipulation is debatable.
Wanda's power to alter reality itself was spawned from the combination of her abilities to affect probability and chaos magic. It must also be noted that Wanda is responsible for the resurrection of Mad Jim Jaspers which happened when her powers altered reality. As such, she may possibly be on par with the White Phoenix of the Crown. Because of her ability to warp reality, Wanda is one of the most powerful Marvel beings.