Apocalypse pretty much explodes every monster on the screen, damaging them all pretty heavily.

I think the only way you can use apocalypse legitly in D1 without the hellfire expansion is to use a scroll or staff. In Hellfire, I remember you get a book of apocalypse eventually, but that only gives you level 1 apocalypse.

The level 1 version of apocalypse isn't all that great really. The only real bonus of it is you can hit shit that you can't even see, like if there are a bunch of monsters behind a wall or something. It literally hits everything on the screen regardless of anything.