As gogtrial said and explained, Wine is the way to go. There is a little free program called porting kit that can help you to install some (not all) of the windows games on your mac. And the good thing for you, Diablo is also supported and there is a tutorial video how to install it. But be aware, it's not an official program and there can always be problems. Just watch the video and then decide if you want to go that route.
As far as I understand it, SheepShaver is a way to make classic (PowerPC) Mac games run on modern machines (both Linux and current Mac OS X)
GOG offers a windows version of Diablo 1. This is the classic game with additions to make it work on windows 7/8/10. GOG does not offer the original game as it came on disk, and definitely not the classic Mac version of that. So no, SheepShaver is unlikely to make the modern windows Diablo 1 work on Mac OS X.
However, there are some reports
about getting Diablo playable under wine
on Linux, and those should work just as well on Mac OS X. From the looks of it, it won't be usable out of the box, but the steps to get it all to behave look doable (as long as you don't shy away from experimenting a bit, given that you'll likely be unfamiliar with what some of the terminology means for what you have to do).
edit: reading a bit further, it seems that DevilutionX is likely to be a better solution
to get GOG's version to work.