Dark Sacrament

As part of casting this spell you must expend four hit dice or the spell automatically fails. If you do, make a melee spell attack against a creature within 5 feet using the material component of this spell. On a hit, roll the hit dice expended casting this spell plus an additional 4d8 and deal necrotic damage equal to the result. If this damage causes the creature to be reduced to 0 hit points, it immediately dies and you gain one of the following dark blessings of your choice. 

• Unassailable. You have advantage on all saving throws. 
• Unbreakable. Your size increases by one category (from Medium to Large for example), you gain a number of temporary hit points equal to your Constitution modifier (minimum 1) at the start of each of your turns, and your weapon attacks deal an additional 1d4 damage. 
• Unerring. Your proficiency bonus increases by 2. 

Your dark blessing ends after 10 minutes or when you are reduced to 0 hit points, whichever happens first. When you take radiant damage while you have a dark blessing, you take an additional 1d4 damage. 

At Higher Levels. When you cast this spell using a spell slot of 5th level or higher, the melee spell attack damage increases by 1d8 for each slot level above 4th.