Abyssal Persecution

This is a 10th level spell.          Casters: 1

You channel horrific dark magic from the deepest and most cruel depths of the Abyss, calling forth a horned devil with the creature subtype of "demon" instead of "devil", 190 hit points, +2 to hit and to damage, and has a longsword instead of a fork: +12 to hit, reach 7 ft, one target. Hit: 17 (2d8 + 8) slashing damage plus 5 (1d6 + 2) necrotic damage. When the demon is summoned, each hostile creature within 40 ft of it must make a Dexterity, Constitution, or Wisdom saving throw (creature's choice), taking 2d8 necrotic damage on a failed save, or half as much on a success. You have advantage on this saving throw, and are also resistant to the damage dealt if your alignment is chaotic evil.  The demon remains for 30 minutes and is under your control for the duration as long as you maintain concentration. The demon obeys your verbal commands is friendly to your allies while under your control. At the start of each of the demon's turns, you can command the demon (no action required by you and no need to stop concentration on the spell) to do something, and the demon obeys if the command is clearly non-self-destructive. If given no commands, the demon does whatever it wants.

In ancient time of the world of D&D there were no limits for what magic could do. But then Mystra, the goddess of magic, after having to kill herself to save the weave from Karsus' foolishness, decided that such magic was too powerful, reducing the strength of spells an individual could cast to 9th level. Due to the impossibility for a mortal to have 10th level or higher spell slots, there must be a number of secondary casters, each using a 9th level spell slot, to cast it as shown in the Casters. In addition to the secondary casters there are also the main caster who is the person actually casting the spell. This person chooses the targets of the spell and any other choice that must be made to cast the spell and could actually make a spell target some of the secondary casters even if they did not agree to. If a spell requires concentration its only the primary caster who must maintain concentration. Immediately after casting a 10th level or higher spell, all casters must make a DC (9+spell level) Constitution save or suffer from 1 level of exhaustion (10th or 11th level spell), 2 levels of exhaustion (12th or 13th level spell), 3 levels of exhaustion (14th or 15th level spell), 4 levels of exhaustion (16th level spell), instant death (17th level or higher spell).