The rest of the team hung back to talk to one another as usual at the end of practice, but Harry headed straight back to the Gryffindor common room, where he found Ron and Hermione playing chess. Chess was the only thing Hermione ever lost at, something Harry and Ron thought was very good for her.
(Harry Potter and the Sorcerer's Stone)
‘Something’ looks alike to complementizers, for example ‘that’ or ‘what’ or ‘which’. But I can’t find this usage in dictionaries. What role does ‘something’ take in this context?