A little torn on this one. Obviously, Blizzard took the name Diablo and built something around it...but they didn't create it.
here's the "Diablo" Disambiguation page
https://en.wikipedia.org/wiki/Diablo
Just for character names alone you have this:
Diablo (Disney), a raven in Sleeping Beauty
Diablo (Marvel Comics), a Fantastic Four villain
El Diablo (comics), several fictional characters from DC Comics
A character in the Diablo video game series
-a playable character in the video game Heroes of the Storm
A character in the video game Primal Rage
El Diablo, a superhero in the video game Freedom Force
El Diablo, a character in The SpongeBob Movie: Sponge on the Run
There's also the fact that "Diablo" just means "Devil" in Spanish
So Blizzard might have a hard time winning this case.