Time for a warlock that is based on knowledge?
None know what you found, but it offered you knowledge. Dark, fel knowledge that no one else knew. Knowledge is power and power corrupts, and this creature offered you enough knowledge to give you all the power you ever need. May you stay strong enough to not be completely corrupted by it.
Starting at 1st level, you gain a spell book and two 1st level spells and two cantrips from the wizards spell list. You can add more spells to the book as a wizard does from the wizard spell list. When you take a short rest you can prepare as many spells are you are able, but you must prepare individual uses of each spell. To cast a spell multiple times, you must have prepared it multiple times. You are still limited to your maximum spell level based on your level.
Learn Through Experience
Starting at 6th level, if you succeed against a save from the wizards spell list that spell appears in your spell book. You need not be able to cast the spell yet to add it to your spell book this way.
Starting at 10th level, up to a number of times per day equal to your Intelligence modifier, when you cast a spell you can gain advantage on the spell attack or cause one target to suffer disadvantage on a save against the spell.
Starting at 14th level, the dark master blesses you far beyond others that it controls and other warlocks who follow lesser masters. You can prepare one additional wizard spell that you know that is at the maximum level of a wizard of your level per day. You must have this spell in your spell book.