The Great One is an otherworldly being who can only enter the world of mortals through sleep. He is the night sky, the stars pinning him there in place, the moon his ever-watchful eye. Those who worship the great one view him as a god, and whether he is could be its own debate. He grants those who follow him the power to call upon him through dreams and allows them to cast nightmares upon one's enemies. Many of his followers use spells such as sleep as a useful way of accessing his power, however, they also use spells that compel creatures to do what they ask. Any way they achieve placing a creature to rest, their patron will be sure to make sure they never wake up again. 

Extended spell list

1st - Sleep, Command
2nd - Calm Emotions, Moonbeam
3rd - Fear, Glyph of Warding
4th - Phantasmal Killer, Dominate Beast
5th - Geas,  Commune

Invoke Nightmare

Starting at level 1, the great one blesses you with their power to infest the nightmares of your enemies. Whenever a spell or ability you use would cause a creature hostile to you to fall asleep, that creature must make a Wisdom saving throw. If they fail, that creature takes 1d4 psychic damage at the start of their next turn. This damage does not cause the affected creature to wake from its sleep. At the end of the creature’s turn, they can make another Wisdom saving throw, ending the effect on a success. This ability gains 1d4 at levels 5 (2d4), 8 (3d4), 11 (4d4), 14 (5d4), and 17 (6d4). 

Waking Nightmare

At 6th level, you have learned how to trap your enemies in their worst nightmares, even while they are awake. When you use this ability, choose a number of targets equal to your spellcasting modifier. Those creatures must make a Wisdom saving throw against your spell DC or fall into a waking slumber. These creatures, while still conscious and able to take action, are considered to be asleep. These creatures cannot take reactions and can make another Wisdom saving throw at the end of their turns to end this effect. You may use this ability a number of times equal to your proficiency bonus.

Conjure Nightmare

At level 10, you gain the ability to bring the nightmares of your enemies to life. Whenever a sleeping creature takes damage from your Invoke Nightmare ability, a Nightmare clone of the creature is created 10ft from them. The nightmare has the stats and abilities of the sleeping creature as long as their challenge rating is less than or equal to your level. The nightmare does half the damage and dies when the creature is no longer sleeping. The Nightmare attacks a random creature it can reach without moving 10ft from the sleeping creature.

Exploit Nightmares

At level 14, You now not only can control the nightmares of your enemies, but weaponize the nightmares of your allies as well. Whenever one of your allies becomes unconscious, you summon a nightmare clone of them. They control the nightmare, and the nightmare shares the stats of the ally. The nightmare deals half damage and dies when the ally becomes conscious or dies.