From a legal perspective, nightcore is definitely a derivative work that infringes upon the original copyright holder's copyright.
However, what nightcore does is it essentially attempts to change the song just enough to avoid automatic detection by Content ID. That doesn't make it any more legal, mind you. But that explains why so much nightcore still persists.
Copyright isn't just about attribution, so just linking to the original in the description isn't enough.
Copyright is about compensation -- so really, you'd need to be paying the original copyright owner royalties to use their song in a derivative work. Most nightcore people do not do this, so they are definitely illegal.
