10 Most Powerful X-Men Villains
4. Onslaught
You know that saying, "when you play with fire, you get burned?" That little idiom explains the existence of Onslaught, one of the deadliest entities in the Marvel Universe. He was created when Charles Xavier attacked Magneto and shut down his mind with his telepathic powers.
When he did this, it rendered his enemy catatonic, but the darkest and most evil aspects Magneto's mind escaped to Xavier's where it merged with the darkest aspects of his, and this new persona eventually emerged as Onslaught. He is a psychic entity given form, and his superpowers have been described as "Psychic Omnipotence."
He learned to manifest his own body, and eventually managed to bring his enemies to the astral plane where he psychically defeated them. This included Jean Grey and Charles Xavier, both of whom are two of the most powerful telepaths in the Marvel Universe.
He went on to possess the abilities of Nate Grey and Franklin Richards, which essentially made him an omnipotent god, and well above the standard definition of an Omega-level mutant. He also possesses vast superhuman abilities, and once pulled the Crimson Gem of Cyttorak from the Juggernaut's body. In his true physical form, he is a mass of pure, psionic energy capable of wanton destruction, and completely immune to all forms of attack.