https://www.fantasyflightgames.com/en/news/2019/3/25/before-the-black-throne/

QuoteThe Call of the Void
As you venture far into space, breaking away from the rest of humanity, other connections grow inexplicably stronger—some cards in this Mythos Pack feature the new bonded keyword, keeping them specifically linked to other player cards. These bonded cards do not have levels and they are not available as options when deckbuilding. Rather, the cards they are bonded to will summon them into play. For example, you can only execute a Blood-Rite (Before the Black Throne, 317) if you have the Occult Lexicon (Before the Black Throne, 316) that teaches you how to perform it. While the lexicon is an asset that you must choose to add to your investigator deck, the event that it summons is not a part of your investigator deck and does not count toward your deck size. In the case of Occult Lexicon, adding this single card to your deck could give you access to three copies of Blood-Rite without removing any other valued cards. Rather, these bonded cards will simply be set aside, ready for the moment you spring them into action!
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Color me interested.
