You've made a pact with Sybill the Thief Goddess  

Level 3: Necrotic Spells

Whenever you cast a spell that deals damage, you can choose to change the damage type to necrotic

Bend Shadows

You learn to bend shadow to cover yourself and your allies making it easier to Sneak without being noticed. You Learn the Spell Pass without trace, and always have it prepared. You can cast this spell or the invisibility spell at will without expending a spell slot a number of times equal to your Proficiency bonus per long rest. You also gain Expertise in the Stealth, and Deception Skills. 

Level 3: Shadow Spells

The magic of your patron ensures you always have certain spells ready; when you reach a Warlock level specified in the Shadow Spells table, you thereafter always have the listed spells prepared.

Fiend Spells

Warlock Level	Spells
3	Blindness/Deafness, Darkness, Invisibility, Mirror Image
5	Blink, Nondetection
7	Greater Invisibility, Phantasmal Killer
9	Mislead, Creation

Level 6: Separate Shadow

you rend your opponents shadow from them as a shadow under your command. Once per long rest when you hit a creature with a spell attack that deals necrotic, or force damage. You can use your bonus action to rend the targets shadow. When you do a shadow appears directly behind the target (Shadow stat block can be found in the basic rules) The shadow operates as your ally and takes its turn either directly before or after you (your choice) on its turn it uses its strength drain attack against the closest hostile creature unless otherwise directed by you. The shadow lasts for one minute or until it hits 0 hit points or is dismissed by a bonus action. The shadow operates the same as the basic rules Stat block except it is not sensitive to sunlight and a creature it kills with its strength drain attack becomes a shadow in 1d4 rounds. Any additional shadows are also under your control (up to a max of 5) and are dismissed when the original shadow is dismissed or a minute expires, but not if the original shadow drops to 0 hit points. 

Level 10: Shadow Guard you

You gain resistance to Both Necrotic, and force damage. In addition you can now hide or disengage as a bonus action. 

Level 14: Shadow Sphere

You emanate a 20ft sphere of shadow covering yourself and your allies, all allies in your sphere gain dark vision to see through your shadow, and are lightly obscured giving them half cover. In addition you gain true sight in a range of 60ft.