Board Thread:Legitimate Canon Questions/@comment-118.136.31.14-20141213140029/@comment-4091815-20141214015827

There is little to be found in the canon on this point.

We were told Derek's entire Season 4 plot (slowly losing his werewolf mojo, becoming human, and dying) was a process of evolution.

Since this was revealed in the final episode of the season, there is no further information yet available.

As for what Kate did to him (regressing his body and mind) - it is implied that it had something to do with the power that resides in the temple and the Jaguar God. However, nothing was clearly stated on the issue.