i faced a Kyurem in the giant chasm and it fainted and I got a DNA splicer so when does reappear and how doi catch it
Just rebattle the Elite Four. In every game since Platinum, if you beat the Elite Four again, the Legendary will reappear.
You have to battle the elite 4 and the legendary will reappear for your bidding
Rebattle the Elite 4, and he will reappear.
If you do happen to defeat the Legendary, then all you have to do is defeat the Elite Four again and then it will reappear. I found this out the hard way.