Cult of Dread

You are/were a member of a dread cult with access to a patron of some sort that you can call upon for an unholy power that comes with a high cost. 

You manifest an aspect of your patron’s dreadful power. As a bonus action, you transform and gain the following benefits:

• You gain temporary hit points equal to 1d10 + your class level.
• Once during each of your turns, when you hit a creature with an attack, you can force it to make a Wisdom saving throw, and if the saving throw fails, the target is frightened of you until the end of your next turn.
• You are immune to the frightened condition.

You can transform a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a long rest.

The appearance of your Form of Dread reflects some aspect of your patron. For example, your form could be a shroud of shadows forming the crown and robes of your lich patron, or your face might transform into bat-like features due to your vampire patron.

While in this form, you do not need to eat, drink, or breathe, and you can use an action to cast spells, using your own life force. This can be any spell on the wizard spell list, or that makes sense with the patron you choose, but any checks made to cast spells of a higher level than you can normally cast are made at disadvantage, as are Con checks to keep spells active. 

This form takes an unholy toll on your soul, stealing some of your life force each time you use it. Every time you use it, roll 1d20 per each minute of use, with disadvantage on one roll per each spell cast. On a roll lower than 10, roll percentile die. You begin with a 10% chance that you will drop to 0 HP and fall unconscious from loss of life force, resulting in you having to make death saves. Increase the percentage by 10 for every failed d20, and by 5 for every level of a spell that is higher than your class level can cast. This change to the percentage is permanent, with no way to undo it, short of a wish spell. Regardless of number rolled, you gain one level of exhaustion once your form drops, plus one level for each failed "soul" save.