How do I sync it with the original Diablo?

Belzebub got a different engine and actually can run 16:9 including zooming in and out, which is not possible on the Classic and Hellfire version. As i see it, it is a separate game on GOG because GOG was already merging it, so there is no need to merge it with a old version, it is standalone. Perhaps the saves are compatible with each others, i dunno. Works fine, every version i was testing, including Belzebub.

when I hit the play button this happens: