I seriously doubt that DD uses the actual "Diablo" font even though it may look very similar. I have several fonts installed that also look close to the Diablo font but are not. ( I use them on covers )

I can't answer a question about the Larian Studios being sued as I have no insider knowledge of such things. I do know publishers / devs go to great lengths to avoid using any copyrighted material without having permission to do so. Most likely the fonts used in DD are specifically designed for the game by the devs or the design was sub-contracted to a professional font designer who would avoid any use of copyrighted fonts in his font creations.

The "magic", "medieval", "Gothic" fonts are very popular for RPG fantasy games. Take a look at some of the free fonts offered by dafont.com. Look under the Gothic\Medieval themes and else where as the "ancient" fonts are scattered about.

Some folks make "near" fonts for some popular games but they are not exactly the same or . . the rights owner has released them into the wild or . . . people just start using them without permission which is something a publisher would not do generally. Blizzard would have sued for sure and the game would have been pulled years ago most likely.

The Witcher fonts were actually released in a "Fan Kit" but I have not seen the actual Diablo fonts in the wild other than looks-like attempts since Blizzard keeps a tight grip on their stuff. . . =)

Edit: You got my curiosity roused so I had to look. Maybe the n and a are slightly similar but I would say that the games use two different fonts IMHO. . . =)