Diablo 1 HD Hellfire Mod | Awake

Just killed diablo on torment with my mage.
but something strange happened:

i almost killed him and then he reappeared with full health and the strange "mumbling sound" got played. i had to kill him "again" but this time his attacks were really weak...dont know what that was

Likely you ran out of time on the Map of Stars, then Diablo reset himself. You then just got lucky and still kicked his ass.

I've had this happen to me on Nightmare I think it was, and it definitely wasn't because I ran out of time on the map of stars. I still had a good 30+ minutes. And Diablo didn't receive any affixes, and as OP said, his attacks were super weak. Gotta be a bug.

PS grats on the torment win