Let's say I'm an 11th level Pact of the Tome Warlock with Book of Ancient Secrets. I'm able to learn spells to cast as rituals from any source, I can transcribe spells up to half my level rounded up, and have just gotten my 6th level Mystic Arcanum.
I do not actually have 6th level slots, but ritual casting doesn't use spell slots anyway, and I am capable of casting 6th level spells (or at least one). Am I able to transcribe a 6th level ritual spell, or am I capped at learning 5th level rituals?
Best Answer
Yes, but it's not a big deal
There are only two spells that are above 5th level and have the "ritual" tag; Forbiddance and Drawmij's Instant Summons
Let's look at the wording of the Invocation:
So you start with two spells from any class list, so long as they are rituals. As you adventure, you can also transcribe other ritual spells, so long as their level is half of your warlock level, rounded down. Not the level of spell you can cast. So at 12th level, you can cast 6th level ritual spells, so long as you have acquired and transcribed them into your Book of Shadows.
Additional, you can "cast a warlock spell you know as a ritual if it has the ritual tag." But once again, this list is very limited; seven total if we include all Warlock sub-classes:
And it works for Arcanum, too
If some future rule change adds either Forbiddance or Instant Summons as an Arcanum, or they make a new sub-class with a new 6th-level ritual spell, those work too.
The wording of Arcanum states:
So Arcanum are warlock spells, and the Invocation states that the character can cast ritual warlock spells as rituals.