Prerequisite: Pact of the Tome feature
You can now inscribe magical rituals in your Book of Shadows. Choose two 1st-level spells that have the ritual tag from any class's spell list (the two needn't be from the same list). The spells appear in the book and don't count against the number of spells you know. With your Book of Shadows in hand, you can cast the chosen spells as rituals. You can't cast the spells except as rituals, unless you've learned them by some other means. You can also cast a warlock spell you know as a ritual if it has the ritual tag.
On your adventures, you can add other ritual spells to your Book of Shadows. When you find such a spell, you can add it to the book if the spell's level is equal to or less than half your warlock level (rounded up) and if you can spare the time to transcribe the spell. For each level of the spell, the transcription process takes 2 hours and costs 50 gp for the rare inks needed to inscribe it.
Prerequisite: 9th level, Pact of the Tome feature
A new page appears in your Book of Shadows. With your permission, a creature can use its action to write its name on that page, which can contain a number of names equal to your proficiency bonus.
When any creature whose name is on the page is reduced to 0 hit points but not killed outright, the creature magically drops to 1 hit point instead. Once this magic is triggered, no creature can benefit from it until you finish a long rest.
As an action, you can magically erase a name on the page by touching it.
You have advantage on Constitution saving throws that you make to maintain your concentration on a spell.
Prerequisite: 5th level, Pact of the Tome feature
A new page appears in your Book of Shadows. With your permission, a creature can use its action to write its name on that page, which can contain a number of names equal to your proficiency bonus.
You can cast the Sending spell, targeting a creature whose name is on the page, without using a spell slot and without using material components. To do so, you must write the message on the page. The target hears the message in their mind, and if the target replies, their message appears on the page, rather than in your mind. The writing disappears after 1 minute.
As an action, you can magically erase a name on the page by touching it.