Armageddon

You unleash the chaotic energies of the Abyss, calling forth a horde of lesser demons. These demons are mindless creatures who are not strong, particularly intelligent or strategic, each possessing 10 hit points but they make up for their lack of cunning with sheer brutality and tenacity.They are bound to your will, attacking any creature they perceive as a threat, excluding the caster.

As long as you maintain concentration on the spell, you can continue to summon an additional horde of 10 demons every turn. 
Once the spell ends, any remaining demons vanish back into the Abyss from whence they came.