I vaguely remember reading how some bodhisattva out of compassion went to some hell and then transformed it into a pure land. Does anyone know that story? About which bodhisattva it is? And what's the source for it...
Found it, it's a story about Guanyin, the Chinese version of Avalokitesvara. The story says that she was a woman named Miaoshan, and when she died she went to hell (depending on the version either bc of some magic tiger, or because she took the bad karma of her executioner on herself), and there by her music or simply her presence she transformed it into a heaven. And then Yama kicked her out of the hell plane so that she doesn't totally destroy (/transform) it. It's mentioned on the wikipedia page from Guanyin.
If you are just talking about a bodhisattva which goes to hell and help sentient beings, that would be Bodhisattva Kṣitigarbha. I haven’t heard of a bodhisattva who transformed a hell into a pure land yet.