Pact With the Black Mage

The Black Mage Pact gives additional spell slots and gives you access to more spells from other classes at a loss for alternative forms of combat, similar to a Wizard or Sorcerer. Additionally it gives you access to the Archive, a psychically accessible well of near-infinite mystical knowledge, containing every form of magic and every spell that has ever been made or cast. However, navigating this takes a significant level of intellect to properly traverse, and subsequently use properly.

The Archive is Opened

The Warlock is given full knowledge of all level 1 spells and cantrips across all classes, however will only be able to cast the ones they prepare in their spell list at any given time. 

The Warlock also learns how to ritually cast Identify on items. This does not count against spells you know.

Mage's Peerless Affinity

The Warlock gains a second spell slot table and a second separate spell list, exclusive to themselves. These are referred to as Black Mage Spell Slots and the Black Mage Spell List. Black Mage Spell Slots can only be used by spells on the Black Mage Spell List, and vice versa. The Black Mage Spell Slots are always half the level of the current max spell level of the Warlock's standard spell table, rounded down. They also have the same amount of Black Mage Spell Slots as their standard spell slots. The Black Mage Spell List size is equal to half their standard spell list size, rounded down.

The Archive is opened further to the Warlock, and they gain knowledge of all 2nd and 3rd level spells, but must have them in their Learned Spells in order to use them.

(Note: The "Expanded Spell List" part of this feature only lists two spells, those being the alphabetically first and last spells on the list. This is shorthand for all spells between those two, and the spells listed themselves, as not to bloat the selection bar. Just remember the level limitations of your class when picking spells.)

Call of the Black Mage

The Warlock gains a new ability, at the start of their turn, they can choose to activate it. For the next 2 turns, they can cast any 3rd level or lower spell, whether or not they know it, without using materials (of a gold cost lower than 10gp) or a spell slot, on both their Action or Bonus Action.

The Warlock also gains access to knowledge of all spells of 4th and 5th level, however in order to recollect information about magic of 5th level and higher they must make an Intelligence check of 10 + the spell's base level in order to recover that information. This does not apply to Learned Spells.

Power Overwhelming

The Warlock gains access to a group of powerful Black Mage Signature Spells, extremely powerful one-use casts that are often far more powerful than their standard variant. At the start of each day, a Warlock must choose only one of these spells to have ready, and they must notify their GM of what spell they are choosing. If a Warlock chooses to use this spell, they cannot activate their Call of the Black Mage ability, and if they activate their Call of the Black Mage ability, they cannot use their Black Mage Signature Spell.

Additionally, the Warlock gains the entirety of the Archive as knowledge, from 6th to 9th level spells, however they now have to make a Constitution Save whenever they try and recollect a spell level 8 or higher (this does not apply to Learned Spells), with failure resulting in the Warlock not recovering the information and being Stunned for 6 seconds, or until the start of their next turn.

Finally, the Warlock's Call of the Black Mage ability lasts 2 additional turns and can now cast up to 4th level without need for consuming materials (worth 10gp or less) or spell slots.