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Apocalypse is a very useful spell, but and it hits for physical damage so can't kill any monster, even triple immunes. It can not be learned in Diablo, so must be cast from staves of or scrolls. (In Hellfire it is learnable, but you can only get the books from killing Na Krul, apoc . The damage is much weakened in damagelowered, costs ungodly mana, and doesn't work over walls, so it's not really practical there.) Its great benefit in Diablo is that is no longer works over wallsout of line of sight, and the mana cost is a physical attackvery high, along but the lines "kills anything" nature of stone curse and holy bolt, though both of those spells are useless the spell makes it very effective against certain monsters. In other words, nothing is immune or resistant to apocalypse, including your character, though as a physical attack it can be blocked. Only Diablo casts apoctriple immunes.

The great benefit of Apocalypse in Diablo is that it works over walls and damages anything and everything on the screen. In other words, nothing is immune or resistant to apocalypse, including your character, though as a physical attack it can be blocked. Amongst monsters, only Diablo casts Apoc. A mage with a bountiful or otherwise charge-intensive apoc staff can clear out major portions of the toughest monsters very quickly. My only comment on it is, "Cheap, lame, and wimpy". Mages (or the occasional rogue) who used apoc staves in multi-games I played were almost always those who lacked the skills to kill the monsters for real. Killing tons of stuff over the walls w/o any danger to yourself is not much fun, not very sporting, and not really a tactic I ever saw used much by the higher level chars.  Still, for pure speed, a nice charged up apoc staff left in town and retrieved quickly for some especially tough portion of the game, (perhaps item retrieval after death) can be very useful. I would say don't carry it around normally, though, as it takes the place of 6 full mana pots, and you can do a lot more damage with those pots than with even a bountiful apoc staff in almost every situation, but YMMV.