First, turn on your Steam Deck, press the Steam button and scroll down to Powerpress A, then select Switch to office.
From there, click on the little blue shopping bag icon in your taskbar. This will open the Discover menu, where you’ll find shortcuts to installers for all sorts of apps. Right now we are here for a web browser. So click The Internet, SO Web browsers, and scroll until you find Google Chrome or Firefox and install the one you prefer. Once installed, click Launch high in the upper right corner.
Step 1: Install the Battle.net Launcher
Using your favorite Steam Deck web browser, in the search bar, type download.battle.net, and download the Battle.Net installer. Make sure to put it somewhere you can find it easily, like Downloads or your Desktop. Then open Dolphin (Steam Deck File Explorer) and navigate to where you downloaded the Battle.Net installer. Using the touchpad, move your cursor to the Battle.Net Installer, then press the left trigger on your Steam Deck (this works like a right click), then move your cursor down to Add to steam and press the right trigger (it works like a left click).
Step 2: Configure compatibility mode
Now open Steam from your desktop, navigate to Battle.net Installer, and click on it. Of Play menu, hover over the little gear icon and click on it, then scroll down to Properties. Here we want to go to Compatibilitythen click the check mark to Compatibility of forces. Then, from the drop-down list, select Experimental Proton. Then X exit this menu, return to the Battle.net installer play menu and press Play!
Step 3: Install Diablo IV
From there, just log into Battle.net and download Diablo IV just like you would on a desktop computer. Once installed, return to Steam bridge mode, and launch your Battle.net launcher. (It will still say Battle.net Installer, but it works like the Battle.net launcher on a desktop), select Diablo IVand go save the day!
Or do what I did and wander around a bit, avoiding the main story at all costs and enjoying the sights (blood) and sounds (blood) of Sanctuary!