Apocalypse is the ability you use to pop 4 Festering Wounds, summoning an Army of the Dead ghoul for 15 seconds per wound.
If you look in your spellbook you should find another ability called Army of the Dead, which uses 3 runes to summon a number of ghouls that attack your target.
When a trait reduces the cooldown of Army of the Dead, it refers to this second ability.
for added value info
the abomination talent that replaces army of the dead in pvp scenarios also summons a magus ghoul if u have the trait, making the talent even more interesting.
Army of dead summons 8 ghouls while apocalypse summons max to 4,army is 8 min cd while apocalypse is 1.5 min.
How on the earth is that the same to you?