The dark eclipse

This spell requires you to go to the deepest depths of the 9 hells in order to retrieve the horn and scale. First, you carve the horn so that you can blow a deep sound out of it. Next you smash the blood ruby over the scale of the balor. Finally you chant the names of all layers of hell 3 times. If all is done correctly it should cause an eclipse that lasts for 666 days. 

When the ritual is complete, an eclipse covers the sun for 666 days causing eternal night, during this artificial night, all non evil aligned creatures take 1d6 psychic damage and are stunned. After 5 days 2d20 demons from the first layer of hell are released, after another 5 days of casting, 2d20 demons from the second layer are released and so on and so forth until 45 days have passed when Asmodeus himself is released. During the time that the demons are released devils are also released after 3 days of casting, 1st level devils are released, along with lemures, after another 3 days, 2nd level devils are released and so on and so forth. After casting this spell, you instantly gain a flying speed of 200ft and a walking speed of 150ft as well, you also gain the ability to cast all conjuration, necromancy, evocation, enchantment, transmutation and abjuration spells permanently. You also gain the following action:

hellish chains; 1d10 flaming chains sprout up from the ground stabbing anyone of your choice within a range of 20ft, they take 4d6 piercing damage, 2d6 necrotic damage, and 2d6 fire damage, this damage can not be reduced in any way. The creature gains the grappled condition. The caster of the spell can use another action to pull the creature through to the 9th layer of hell or they can use the action to strangle the target dealing 1d6 bludgeoning damage every turn, the target cannot breathe and is incapacitated, every round that they are still strangled they take 10 damage until death. After killing a creature with the hellish chains, they rise up as a skeleton (see tab in monster manual) with the same amount of health as the targets hit point maximum, the skeleton is under the casters control, the skeleton automatically succeeds all saving throws that attempt to transfer control to another creature, you can have as many skeletons as you want. Hellish chains can target all creatures that are huge or smaller.