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.
I wasn't able to find anything existing about this already, so I apologize if this is a known bug. In the Character Builder, it shows the option to select three spells, but the invocation should only allow you to select two 1st-level spells. It's easy enough to just ignore the 3rd dropdown, but probably shouldn't be displaying it to begin with unless I'm missing something.
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.
That extra dropdown is how you would add the next spell to your book. Every time you add another spell another dropdown will appear for the next spell. (Until you’ve learned them all.)
I wasn't able to find anything existing about this already, so I apologize if this is a known bug.
In the Character Builder, it shows the option to select three spells, but the invocation should only allow you to select two 1st-level spells. It's easy enough to just ignore the 3rd dropdown, but probably shouldn't be displaying it to begin with unless I'm missing something.
You have indeed missed something:
That extra dropdown is how you would add the next spell to your book. Every time you add another spell another dropdown will appear for the next spell. (Until you’ve learned them all.)
Creating Epic Boons on DDB
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Ahh that makes sense now. The future new spells didn't click for some reason, thanks for clearing it up!
Happy to help
Creating Epic Boons on DDB
DDB Buyers' Guide
Hardcovers, DDB & You
Content Troubleshooting