Diablo uses a sprite based isometric game engine that Blizzard made themselves which they probaly keep in a vault somewhere (they ain't gonna sell it).

Now graphics. If you look at the .mpq data files on the CD (I think its the CD, haven't checked in ages), you can tell that they are made in a 3d app. I'm not sure what they used backed then, but since Blizzard likes to use 3ds max, that was probaly it.

To put custom details inside Diablo isn't too difficult. All you need is a .mpq exporter, importer to open and modify the data files. You can find them diablo modding sites. The hard part is actually making content in the same format which diablo uses. Your .mpq exporter, importer should have viewers for the content meaning that if you look hard enough, you should find somewhere programs that take raw content and encode into diablo format.

The diablo content are just sprites, the characters, monsters, objects and all are just a collection of still frames put together with control data so that the program can display this combination of frames when this action occurs. To do this in Maya you probaly have to model a character and then animate it to do something like walking, but make sure that the framerate matches that for diablo. This is the same for objects and backgrounds (I think), but keep in mind some sprites are just static.

I can drone on and on about this, but there are two last major things you have to know. Diablo uses custom palletes for colour. You will have to use their palletes and make your colours match theirs or if your really up to it, make your own palletes, though I don't know what format they are in. Also, when looking for .mpq viewers, make sure you get the one for your version of diablo, not starcraft.

bakkily > 'I heard flash was....but have no clue...'

Who said that?