The Grimoire of Secrets

You have made a pact with Grimoire of Forbidden Knowledge. Perhaps it was once owned by a great warlock and part of his soul and power remains trapped in the book. The book whispers of the dark secrets that it contains luring those who seek its forbidden power to it. It bestows upon them it's learnings on you.

Lower Mystic Arcanum

Starting at 1st level your patron bestows upon you forbidden knowledge not often known by those of your persausion as an arcanum. Choose one 1st-level spell from any spell list as this arcanum. This spell counts as a warlock spell for you.

You can cast your arcanum spell once without expending a spell slot. You must finish a long rest before you can do so again.

When you level up you can switch this arcanum for another spell of your choice. This spell can be replaced with a 2nd-level spell or a 3rd-level spell of your choice from any spell list when your warlock spell slots reach the corresponding level.

For example, when you reach the 5th level you could learn the fireball spell with this feature.

Forbidden Guidance

Starting at 6th level, you learn the secret of manipulating your spells to ensure they hit. When you make a spell attack, you can use this feature to add a d10 to your roll. You can do so after seeing the initial roll but before any of the roll's effects occur.

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

Eldritch Defenses

Starting at 10th level, you have resistance to magic. You have advantage on saving throws against spells and other magical effects.

Forbidden Arcanum

Starting at 14th level, the Grimoire imparts even greater secrets to you, filling your mind with forbidden knowledge.

Whenever you learn a spell due to the Mystic Arcanum feature you may learn another spell of the same level from any spell list. If this spell is not from the warlock spell list, it counts as a warlock spell for you. You may only cast 1 of the spell learned this way with the Mystic Arcanum feature per long rest.

For example at 14th level you have already learned a 6th-level and 7th-level warlock spell due to the Mystic Arcanum feature. You may select another 6th-level and another 7th-level spell from any spell list. However, you may only cast 1 6th-level spell and 1 7th-level spell per long rest.