Chant to the Old One

This spell is a blessing rarely given to mortals by the Great Old One, it implies great expectations from him. The spell requires a special artifact to fully function, so even if you manage to learn this spell, it won't work properly unless you have the mark of a Mark of the Old one in your possession.

Casting without a Mark:

When you cast this spell without having the a Mark of the Old One in your possession the spell deals 1d8 psychic damage to every hostile creature in a 20ft radius centered on you and after you cast it you can only cast cantrips for the duration of the spell. This is because even though you've learnt the spell it doesn't mean the Old One has recognised you as one of his vassals yet. 

Casting with a Mark :

When you have the Mark of the Old One in your possession the spell's true potential comes to light. 

• When you cast the spell using a spell slot, the spell must be casted as an action, and it deals 5d8 psychic damage to every hostile creature within a 20 ft radius centered on you. Each creature must succeed a WIS saving throw against your Spell DC, on a success they take half the damage. 

• When you cast the spell using the Mark you can cast it as a bonus action, the spell deals 5d8 psychic damage to every hostile creature within a 15 ft radius centered on you and all of those creatures becomes vulnerable to a damage type of your choice. Each creature must succeed a WIS saving throw against your Spell DC, on a success they take half the damage only.