The fantasy anime series Bleach focuses on spirits and the afterlife for its themes and combat system. In this universe, it's the job of black-robed Soul Reapers to escort dead souls to the Soul Society and slay monstrous Hollows. All this is done with soul-cutting swords, or zanpakuto, which have a stunning array of powers and abilities.
Ichigo's own sword, Zangetsu, is rather straightforward as an aggressive melee type. However, there are also kido-based zanpakuto with magical powers -- this includes ex-Captain Sosuke Aizen, the arch-villain for the bulk of Bleach's story. His zanpakuto's incredible power allowed him to carry out wicked schemes, and it's a power all fear and none can escape.
What Sosuke Aizen's Zanpakuto, Kyoka Suigetsu, Is Capable Of
Sosuke Aizen's sealed zanpakuto, Kyoka Suigetsu, is a fairly regular katana with a green hilt and an elongated, hexagon-shaped handguard. Aizen himself is well-known for his swordsmanship, able to cut down nearly any foe with his blade and remarkable speed. But upon hatching his bold scheme to rule the universe, he needed the power of deception and illusion to get his way -- that's where shikai comes in. Aizen's shikai, Kyoka Suigetsu, is like a trickster god, and no one can fool it.
The shikai's release command is "Shatter, Kyoka Suigetsu," and once this happens, anyone who looks at the sword will fall under its spell without even realizing it. Aizen described this power as perfect hypnosis, and it's easy to see why. Kyoka Suigetsu will fool all of the victim's major senses -- sight, sound, touch, taste and even smell -- and make them experience any false reality that Aizen wishes.
The illusions are convincing and, as far as Ichigo and his friends can tell, there is no way to break free of Kyoka Suigetsu's absolute power. There isn't even a time limit on this -- it will continue for any length of time, years or decades if necessary. The only way to break free is for Aizen to once again say "Shatter, Kyoka Suigetsu," and manually lift the spell. Oddly enough, Kyoka Suigetsu does not change its shape when activating or deactivating shikai, as opposed to most zanpakuto.
Aizen's Use Of Kyoka Suigetsu In Bleach
Sosuke Aizen made extensive use of his shikai, though neither his enemies nor the readers/viewers ever saw it coming. About 110 years before the main events of Bleach, then-Lieutenant Aizen of Squad 5 already had his Captain, Shinji Hirako, under Kyoka Suigetsu's power, fooling him into thinking that Aizen was always by his side. It was really a random stranger in his place, while the real Aizen was elsewhere, diving deep into his Hogyoku research. Aizen also used this power to corner Shinji and the other future Visoreds, and used advanced kido spells to escape once Kisuke Urahara and Tessai arrived on the scene.
Aizen later moved his scheme to its next phase, covertly massacring every member of Central 46 -- the Soul Society's ruling body -- while fooling everyone into thinking it was still functioning as normal. Captain Toshiro Hitsugaya eventually found out the truth during the Soul Society arc but was too late to do anything about it, and Aizen struck him down right there in Central 46's chambers. He also briefly used Kyoga Suigetsu to take down Captain Sajin Komamura, and by now, Aizen's shikai was easily established as one of the most remarkable zanpakuto releases in this story arc.
Later still in Bleach, Aizen confronted the assembled Soul Reapers in the fake Karakura Town, who had planned to overwhelm him before he could activate Kyoka Suigetsu's power. Too late -- Aizen had already used it, and Captain Hitsugaya was fooled once again. He tried to strike Aizen down from behind but actually stabbed his friend Momo Hinamori. Only Ichigo had been spared from Aizen's power, and he watched in shock and confusion as Toshiro dealt a near-fatal blow to Momo.
Aizen was briefly freed from the Muken prison to help fight the powerful Yhwach, and used his illusions on the King's own power, known as The Almighty. In the end, though, it was Ichigo Kurosaki who defeated Yhwach for good.