I installed Battle.net launcher in Steam, the .exe file. From there I installed Diablo. Works fine. I had some performance issues and had to add some command line args. Codex/claude code helped me with all of that, it ended up running very smooth (I've since deleted it).
But Steam -> custom app -> battle.net exe -> run it in 'compatibility/proton mode' and you should be good.
But Steam -> custom app -> battle.net exe -> run it in 'compatibility/proton mode' and you should be good.