One of Bleach's smartest villains, Sosuke Aizen carefully and secretly carried out his plans along with his two other followers – Gin Ichimaru and Tosen Kaname – for years before he finally revealing the fruits of his labor. Aizen's move to Hueco Mundo was certainly shocking to Soul Society's elite, but that was just the beginning of a long road for everyone he considered an enemy.
However, Aizen's not just a master schemer -- he's got the powers to back it up. Tite Kubo spent Aizen's early days building up the mystery behind his strength. As a Lieutenant, Aizen was oddly powerful for his rank, both in Kido and combat and, while he was a Captain of the Gotei 13, he one of the strongest, and there was no explanation as to how he got that way. But, all has long since been revealed about Aizen's powers, and there's a lot to cover.
Basic Abilities
Aizen was a Kido Master and his ability to use several different types of Kido spells was actually shocking to the Captain of the Kido Corps – the most skillful kido user in Soul Society. Normally, high-level Kido spells would be too powerful for a normal Shinigami (without an incantation), but Aizen easily used every high-level Kido with no need for incantation at all.
At his base level, Aizen is strong enough to stop Ichigo’s Bankai sword with just one hand, while attacking with the other. He even stopped Captain Komamura’s sword with one hand as well. Despite these feats of strength, Aizen's abilities fell below the radar, thanks to his flashier colleagues.
Zanpakuto Abilities
Aizen’s zanpakuto is named "Kyoka Suigetsu," and its Shikai ability is complete hypnosis. For the hypnosis to be work, the opponent only needs to see the sword once it's drawn. With this, Kyoka Suigetsu is nearly invincible in a fight, though Gin Ichimaru was able to sneakily best its Shikai, but only after several years of analyzing it and gaining Aizen’s trust.
Once the Shikai is activated, Aizen's opponent’s five senses will be under his control, drawing them into an false reality of his own creation. This ability makes him immune to both the Visored and the Captains of Gotei 13, who lean heavily on their perceptions. The Shikai can only be deactivated willingly, at which point it will shatter like broken glasses, revealing the truth to his opponents. Despite its power potential, Aizen's Bankai was never revealed.
Fusing With the Hogyoku
During the battle in fake Karakura Town, Aizen revealed he had fused with and mastered the Hogyoku Orb embedded in his chest, boosting his strength and speed immeasurably. As his power evolved, Aizen transformed into a "chrysalis stage," where his whole body looked as if it were covered in a white cloak. His strength also grew, as he was able to block a powerful attack from Shihouin Yoruichi’s Shunko form with his bare hands. After his chrysalis stage was complete, Aizen transformed yet again, this time gaining a reiatsu (spiritual pressure) so great that it causes anything with little spiritual power to completely disappear.
Aizen's final transformation came courtesy of spite, thanks to Gin's betrayal. This time, he took on a butterfly-esque form, and a new ability --disintegrating his body in one place and reintegrating elsewhere, a slightly more terrifying sort of teleportation. After being beaten by Ichigo, he further morphed into a monster, gaining a Hollow-like head, a third eye and heads on the ends of his "wings." These heads could release "Fragor," a powerful energy able to cause mass destruction.
Aizen is one of the most intelligent and powerful beings to ever be in Gotei 13 as evidently proven through the chaos he creatively caused in the whole of Soul Society and Central 46.