This is for someone who is bored or wanting to just mess around, as Dark Simulacrum is something you get at 85. I suppose it would have more practical value if it were level-appropriate for the mobs you were fighting.
In Hellfire Penninsula, you can find Crust Burster worms. They have a Poison ability they use. If you use Dark Simulacrum on them when they are casting or about to, you get their poison ability. (This is how Dark Simulacrum is supposed to work.) However, instead of simply being able to use the Poison ability once (as with almost all spells), you will have the entire duration of the amount of time the spell is waiting to be used. Using it does not remove the spell nor invoke any cooldown at all. Therefore, you may cast this spell as fast as you can click the button.