It should be well-known by now that Bon Kurei didn’t die at the end of Impel Down, when he sacrificed himself to allow Luffy to escape. Instead, he somehow survived his fight against Magellan, and became the new Queen of Newkama Land on Level 5.5. This is shown in the manga, on the cover of Chapter 666.
How he survived though, is a complete mystery. It seems incredibly unlikely that Magellan, who was strong enough to very quickly defeat Luffy, Inazuma, and Ivankov, would lose to Bon Kurei in a battle of strength. It seems unlikely, too, that Bon Kurei would be able to slip away with his disguise ability. Although he was able to trick the lower ranking guards very easily, it wouldn’t make sense to trick Magellan with other disguises when Magellan already has eyes on him. Magellan is shown to be clearly smarter than Hannyabal and other guards, so I think it would be very unlikely for Bon Kurei to pull off a disguise.
This leads me to one crazy theory: that it was Blackbeard who saved Bon Kurei.
That sounds insane – Blackbeard, though he is granted the same Will of D as Luffy, doesn’t care much for others, and only works to achieve his own goal of obtaining the One Piece and exploration. He also seems to show no remorse for the weak and prioritizes the strong, as seen with his recruitment of only the strongest in Level 6, letting them kill each other to prove their worth.
However, Blackbeard’s new recruit, Shiryu, is the only person that is seen with Magellan’s antidote. My theory is this: Magellan defeated Bon Kurei and threw him back into his cell, probably on Level 3 again. Like Luffy, Magellan left him for dead, covered in poison, for what he did in assisting Luffy in his escape. On their way back up, Blackbeard spots Bon Kurei and recognizes him as one of Luffy’s friends and allies. Blackbeard asks Shiryu to administer the antidote to Kurei and free him from the cell – allowing Bon Kurei to recover and escape to Level 5.5 and become the new Queen of Newkama Land.
Why would Blackbeard do this? Because he’s interested in Luffy’s success. Like Luffy, Blackbeard possesses the Will of D., and aspires to gain the One Piece and seek out adventures in general. The only people he has deemed worthy adversaries so far are other incredibly strong people, such as Ace, (another carrier of the Will of D.), and Whitebeard. Blackbeard has an interest in Luffy because he probably views him as a worthy adversary on his quest to obtain the One Piece, and wants to make the game more fun for himself. Though we all dislike Blackbeard for his violent tendencies and greedy attitude, I believe that Luffy and Blackbeard will share some type of Ash-Gary rivalry, and it’s clear later on in the series that Blackbeard is Luffy’s main antagonist. Therefore, Blackbeard helps Bon Kurei as a sort of “token gesture” that will help Luffy later on, if Bon Kurei can manage to escape. After all, the Blackbeard Pirates and the Straw Hats were the only groups that believed in Skypiea during the Skypiea arc. Two pirate groups that share such similar philosophies will bound to have similarities. Blackbeard probably wants Luffy to assemble the strongest crew he can, so that Blackbeard can have more “fun” playing the game on a “higher difficulty”.