Hogyoku

the hogyoku was an orb that sosuke aizen originaly created but failed to stabilize it, kisuke urahara perfected it but created it to break down the boundaries between soul reapers and hollows, but aizen seems to have found out the hogyoku's true power, to bring forth the users desire.

hollowfy/arrancar transformation
aizens first hogyoku prtotype was originaly designed for the same purpose as what the complete hogyoku does, but aizen sacrificed ordinary souls to try and perfect it, then when he tried sacrificing a hollows soul, it transformed it into the first perfect arrancar. the arrancar had the ability to transform any spirit into a hollow, but powerful soul reapers didn't completely transform, the became the visored's, it was later revealed that the arrancar only had the ability to transform spirits into hollows because of the hogyoku's power to transform.

kisuke urahara's hogyoku was the perfect version, urahara originaly said he created it to break down the boundaries between soul reapers and hollows, but has revealed that he infact created it to counter the effects of the prototype hogyoku and turn the visored back into soul reapers, it didn't have the desired effect but managed to stabilize their inner hollows and allow them to start to control their hollow powers.

when sosuke got ahold of kisukes hogyoku, he told meizu that he could transform the menos into arrancars without telling him about the prototype hogyoku transforming the hollow into an arrancar. aizen makes the menos remove their masks then seems to wrap them in a mache-like bandage all over, he then transfers some of the hogyoku's power into them, then it is only a matter of time before they hatch out of the bandages to show they have fully transformed into an arrancar.

desire manifestation
aizen is the only one who has been seen to use the hogyoku in this way by fusing himself with it and asking the hogyoku to give him allmighty power far greater than that of any soul reaper or hollow, but some might say that this ability has been used before by the arrancars as they wanted to transform into an arrancar and they did via the power of the hogyoku, and with the first true arrancar as he probably wanted to be more powerful.

transformation
sosuke has gone through many transformations due to his will to keep getting stronger and the hogyoku granting his wishes, in his partialy awakened form or the chrysalis stage, his body is covered in a white skin-like material, he has more power in this form but uses the hogyoku for most of his attacks and defences.

after this stage 'wears out' or is finished he goes into his awakened stage but the only difference in the appearence is the head as now his head is shown and his hair is very long, his eyes are now purple and the lines from the hogyoku to the body are purple instead of blue.

the 3rd stage was 'awakened' when gin created a hole in his body to remove the hogyoku, it is predicted that when gin removed the hogyoku, it absorbed some of his spiritual pressure and when aizen killed him and replanted the hogyoku into himself, gins spiritual pressure was fused with aizens, the 3rd form is a more over-the-top version of the awakened stage, he has his long hair, purple eyes, but his white phantom-like figure has become a delicate white robe with a hole in the centre of the chest and a cross holding the hogyoku in the centre of it, but on his head there are a few differences, like the fact that there is no iris in his eyes and his eyebrows are gone, there is also a diamond in the middle of his forehead, and has butterfly-type wings on his back.

his final form is a more upgraded version of the 3rd form, but shouldn't be called the final form as if ichigo didn't defeat him, he would have carried on evolving, his face is black and has no nose, the mouth is wide and has large sharp bone-like teeth, he has a third eye, his body is still white but has a hollow like form to it, his butterfly wings have transformed into sheet-type wings with eyeless, hollow heads on the end of each wing, and has a giant eye in the centre of each wing. his shikai zanpacuto has fused with his black hand and there is now 3 holes in his body, the holes are empty with no cross and are the same size as the hole in his 3rd form but on his chest, the top of his stomach and the bottom of his stomach.

history
the first hogyoku was created by sosuke aizen, he exposed the hogyoku to hundreds of soul reapers and rukongai residents and taking their presence, aizen knew it was incomplete but carried on trying to perfect it. he tried to expose it to a hollow to find out if it worked better, but instead of the hogyoku absorbing the hollow, the hollow toke the hogyoku in and transformed it, the hollows mask started to break and its spiritual pressure increased expotentialy, it had the hogyoku in a hole in the centre of its chest. the arrancar proved to aizen that the hogyoku was more effective on hollows and the hollows where stronger than soul reapers or rukongai residents. the arrancar had the ability to transform any spirit into a hollow by breaking their chain of fate and using the hogyoku's transformation ability, but when it tried to transform some of the captains and luitenants of the 13 court guard squads, it didn't completely transform them, instead, it gave them hollow abilities and partialy transformed them, meizu sanda was also struck but didn't transform, the moment he was struck, he defeated the arrancar, but meizu had an inner hollow because of this incident but it seems meizu forced it to stay dormant. after the arrancar was destroyed, the hogyoku vanished, its reishi was scattered across the soul society.

visored's
due to their hollow powers originating from the hogyoku, they had to be exposed to it, the arrancar which gave them their hollow powers exposed the hogyoku to them to try and transform them. later, the visored's are exposed to kisuke's hogyoku in hopes that it would rid them of their hollow abilities, but failed, it stabilized their powers instead of banishing it.

rukia kuchiki
she was secretly holding the hogyoku, but didn't know it herself

sosuke aizen
aizen originaly created the hogyoku but failed to perfect it. when he got hold of kisuke's hogyoku, he wanted to use it to break the boundaries between hollow and soul reaper, but he had to wait for it's seal to fully break, and when it did he fused it with himself, this allowed him to transform into a being unlike any other, this allowed him to break the boundaries just as he wanted to.

orihime and yasutora
although not official, aizen has said that orihime and yasutora got their powers from being exposed to the soul society (predicted to being close to toba zaraki), their powers are like hollows, but the xcution's have similar powers, it could be said that the xcution's where also exposed to the soul society. the reason orihime and yasutora got their powers from being exposed to the soul society was because the first hogyoku's reishi was scattered accross the soul society.