The Prince of Darkness

You have made a pact with the undying spirit of an ancient musician, who grants you the ability to control your magical technique to suit your needs through furious musical performance in a ritual known as "improvising". Through adopting you as their pupil, they hope to regain their influence on the music of today and revive their ancient art of "jazz". Your patron could be one of the many "Jazz Masters" of ancient times, such as Miles Davis, John Coltrane, Herbie Hancock, Louis Armstrong, Duke Ellington, Charlie Parker, or Dizzy Gillespie.

Copy Spells

Unlike the other subclasses of warlock, the Prince of Darkness does not allow your warlock to pick from an extended spell list. Instead, your warlock gains the ability to learn spells by watching masters cast them.

After seeing another spellcaster cast a spell a number of times equal to twice the spell's level, that spell is added to the warlock spell list for you, provided your warlock spell slots are of appropriate level to cast that spell. You also gain the ability to change your spells known every long rest.

Your warlock cannot learn cantrips from other spellcasters.

Butter Note Avoidance

Starting at 1st level, you learn how to manipulate your situation and turn bad conditions into good ones. When whatever circumstances you are in force you or your teammate to make an attack roll, ability check, or saving throw with disadvantage, they gain advantage on your next roll of that kind.

Bonus Proficiency

You gain proficiency in one musical instrument of your choice, and can provide somatic and verbal components for a spell while one or both hands are occupied by this instrument. You can use the instrument as a spellcasting focus, and while doing so, your spells gain a +1 to attack, damage, and save DC.

Magical Improvisation

Starting at 6th level, you begin to learn to shape your patron's magic to suit your needs. When you cast a spell that does damage, and you are holding an instrument you are proficient in, you can change the damage type to your choice of fire, cold, lightning, or poison, at the sacrifice of damage equal to 10 - your Charisma modifier. 

 

Jazz Master

Once you reach 10th level, your increasing expertise in magical improvising gives you the following benefits.

Arpeggiate: When you cast a warlock spell that does damage to a certain number of targets, you can choose to either halve or double the number of targets. If you do halved the number, each creature takes double the normal damage, and if you double the number, each creature takes half damage. You can do this a number of times equal to half your Cha modifier rounded up (minimum of once), and regain all expended uses after a long rest.

Improvised spell: You can expend a warlock spell slot to cast a series of six cantrips from any spell list (they needn't be from the same list) as an action. These take effect at the same time.

Improved Magical Improv: There is no longer a cost for changing the type of damage for a spell. 

 

Giant Steps

 

Starting at 14th level, your superior creativity allows you to draw power from your arcana to boost the strength of your spells. The spells granted by your Mystic Arcanum ability, as well as ones in the future, now count as spell slots for the purpose of upcasting lower level spells. For instance, if you learned dispel magic as a 3rd-level spell, you could spend your 6th-level arcanum spell to cast it as a 6th-level spell. 

This also works with levels of arcana spells. For instance, if you know etherealness as your 7th-level arcanum, you can spend your 8th-level arcanum spell to cast it as an 8th-level spell. If you did this, and you hadn't spent your 7th-level arcanum spell, you could cast etherealness as a 7th-level spell afterward.

You can also now change your arcanum spells through a long and painful 7-day ritual of entreating your patron through straining improvisation over non-functional harmonic progressions, and you can now learn these spells through your Copy Spells ability.