You have made pact with being of ancient knowledge. The actual nature of this being can vary greatly, it can be a Pact with a sentient library, an ancient spirit, or perhaps even an entity such as Gond or Oghma. This is a pact for a Warlock that seeks something beyond simple power, they seek knowledge, ancient and unknown knowledge that has once and will again exist.
You can leverage this knowledge to know things that your mortal peers would consider miricles, delve the secrets that others do not even know exist, and, most importantly, perhaps even understand the philosphical mechanics of existence.
Expanded Spell List
The ancient entity of knowledge you've made a pact lets you choose from an expended list of spells when you learn a warlock spell. The following spells are added to the warlock spell list for you.
Knowledge Keeper Expanded Spells
|1st||Identify, Tasha's Hideous Laughter|
|2nd||Locate Object, Detect Thoughts|
|3rd||Tiny Servant, Speak with Dead|
|4th||Leomund's Secret Chest, Divination|
|5th||Legend Lore, Skill Empowerment|
When you select this Patron at 1st level, you can access the ancient secrets your patron has granted you. By expending a pact magic spell slot of at least one level higher than the spell you want to cast (or a first level spell slot for a cantrip), you can cast a Divinition, Transmutation or Abjuration spell that does not appear on your spell list. This spell can be from any classes spell list, but counts as a Warlock spell for you when you cast it this way.
Ancient Knowledge is Player Knowledge.
Ancient Secrets is very demanding to a player's knowledge of the spells in D&D - if you haven't played several different classes of casters, it will be challenging to quickly leverage the knowledge at your character's finger tips. Consider playing this subclass only when you feel that you can quickly call to mind an reference enough spells to make full use of these powerful Ancient Secrets.
Note that unlike Wish, Ancient Secrets does not remove the need for component materials or effect the cast time of a mimiced spell.
You can do this a number of times equal to your spell casting modifier, and regain all uses of this feature at the end of a long rest.
Starting at 6th level, your understand of the mechanics beyond magical effects and the cosmic forces are developed enough that if you see a spell casting casting a spell, you can use your reaction to make an arcana check to know what the spell is. If the spell you indentify with this check would force you to make a saving throw, you can make that saving throw with advantage.
Additionally, your comprehension of magic allows you to access magic beyond your power, but not your knowledge. You can cast from a Spell Scroll even if the spell isn't on your spell list, and you can add your proficiency bonus to the ability check to successfully cast from a Spell Scroll.
Single Minded Focus
Starting at 10th level, when you make a concentration saving throw, if you are focusing on a Divination school spell, you automatically pass the saving throw. If you are concentrating on a spell from another school, you can add your spell casting modifier to the result.
Seeped In Ancient Knowledge
Additionally at 10th level, ancient knowledge has seeped so deeply into you, you instinctively draw from the ancient knowledge of your Patron. You gain the eldritch invocation Eyes of the Rune Keeper and it does not count against your Invocations Known. If you already have this invocation, you can select either Beast Speech or Eldritch Sight instead.
Starting at 14th level, you can bind the concepts of all spell casting and the world itself together into a single cohesive concept. As a bonus action, you can replace your spellcasting ability modifier with the sum of your Intelligence, Wisdom, and Charisma modifiers until the end of your turn. You can't use this feature again until you finish a long rest.
Variant Warlock: Intelligence.
Wizards of the Coast has noted that it was originally their intention to make the Warlock an Intelligence caster, and that making Intelligence the Warlock Spell Casting modifier doesn't break anything.
With the approval of your Dungeon Master, consider that a Pact of the Knowledge Keeper warlock might be most thematic playing the Intelligence variant, though it is not by any stretch required.