There is a legitimate open-source port of Diablo, and it needs your own game files.
DevilutionX is an open-source reimplementation of the original Diablo engine, created by developers who wanted the game to run properly on modern systems. It has been compiled for the browser.
It is one of the cleanest examples of the engine-port model. The project ships the engine, you bring the data files from an installation you own, and nothing copyrighted is distributed by the project itself.
Blizzard still sells the original digitally, which makes owning the files easy and cheap.