The Price

You have to offer something of significant value to access the libraries of the Great Old Ones. This sacrifice cannot be Currency based,or Material based . This ability is for flavoring and roleplay, unless the sacrifice that you submit is for example: Eyesight, The ability to hear, etc. These have to be discussed with the DM.

Nyarlathotep's Chaos

At 2nd level, if you are attacked by a spell and the spell, that is cast agaisnt you, is prepared in your spellbook, you can use a reaction to cast it.

Nyog'Sothep's Silence

Also at 6th level, You can cast without the use of vocal components in casting, though it does have to be said mentally.

Yog-Sothoth's Denial

At 10th Level, you can take the 1 AC off of your opponent,who is in sight and has failed a wisdom saving throw against the spell save dc, for the sacrifice of a 3rd level spell slot, any spell slot above 3 will deduct +1, for each spell level above 3rd level, this effect will last 1 minute.

Cthulu's Ward

At 10th level, you have a telepathic ward given to you by the old ones, which gives you resistance to psychic attacks, and gain advantage with frighten and charm effects

Azathoth's Revenge

At 14th level, if you are attacked with a spell, you can use a reaction to cast anyone of your spells you have prepared in your spellbook