You have made a pact with a being with raw arcane power.

Expanded Spell List

The Arcane Master lets you choose from an expanded list of spells when you learn a warlock spell. The following spells are added to the warlock spell list for you.

Fiend Expanded Spells

Spell Level	Spells
1st	magic missile, shield
2nd	knock, rope trick
3rd	glyph of warding, lightning bolt
4th	greater invisibility, polymorph
5th	animate objects, arcane hand

Old Magic

Once per short rest, you may infuse a spell you cast with arcane words of power, that spell always requires a skill check to counter or dispel it, and that check is made with disadvantage.

Arcane Acquisition

Starting at 6th level, when you successfully counter or dispel a spell, gain a pact slot equal to the level of that spell. That slot cannot be used to cast counterspell or dispel magic and the slot disappears when you take a short or long rest

Cantrip Acuity

At 10th level, choose a cantrip from the warlock or wizard spell list with a casting time of 1 action, add it to your known spells if it is not already there. It's casting time becomes 1 bonus action for you.

Deep Knowledge

Starting at 14th level, When you finish a short rest, you may regain the use of one of your sixth or seventh level mystic arcanum spells.

Once you use this feature, you can’t use it again until you finish a long rest.