The caster calls upon his or her patron for aid, and then temporarily gains a surge of their power; transforming the caster into a hybrid between themselves and whatever being is their patron. For ten minutes they gain one of the following benefits in addition for the casters creature type swapping to either fiend, aberration, or fey for the duration (DM decides based upon what it appropriate).

1. The Unspeakable: The caster partially takes upon the form of their patron, turning themselves into a horrid abomination that's very presence is mortifying. In this state, the caster has advantage on deception and intimidation checks, and may attempt to, as an action, spread fear to a creature within 120 feet. The target must succeed a wisdom save against the casters spell save DC. Upon a failure the creature is frightened. The creature may attempt to break free from its terror with another successful wisdom check as an action, but it has disadvantage on this check if it can see the caster. The fear lasts until the creature break free or the spell ends. 

2. The Unseen: The caster begins to rapidly flicker between the material plane and one of the outer planes (based upon the nature of their patron, DM can decide which plane). If the caster is already in one of the outer planes, then have him or her flicker between that plane and the material plane. All attacks on the caster have disadvantage, and if a creature attacks the target with a handheld weapon or ranged projectile and misses, roll a d6. Upon a 1-3, the weapon or projectile is transported to the other plane the caster is flickering between, most likely lose to the wielder forever. 

3. The Undying: The caster, for the duration of the spell, gains natural weapons. These natural weapons could be a variety of things, from long lashing tentacles to an aura of fiendish fire. The caster gains temporary hitpoints equal to his or her warlock level.  The natural weapons summoned have a range of 30 feet, suffer no penalty from being used within 5 feet, and can used with the caster's charisma modifier. The natural weapons deal 4d8 damage plus the caster's charisma modifier and deal either psychic, necrotic, fire, poison, cold, or radiant damage (caster's choice). 

When the effects of the spell end, either by the caster using up its timeframe or loosing concentration, all effects produced on the caster end. In addition, the caster instantly falls to 0 hitpoints but is stable. When unconscious in this state, no magic healing can wake the caster up short of a wish spell, so resting is needed to restore the caster to normal. Upon the caster waking up, they must make a DC 15 constitution saving throw. On a failure they will suffer from one effect of long term madness (DMG Page 260) from the immense mental stress caused by performing the spell. 

When cast at fifth level this spell lasts for 30 minutes instead of 10.