I guess I'll run this by my test party then. My first thought is that Greater Invisibility has a paltry duration, so as soon as they recognize what's happening they will just pull back (SOP when encountering unknown threats), assuming they don't simply kill the thing despite its invisibility. But maybe this will be the one time when a monster actually earns its CR.
Am I to assume the strategy involves detecting the party's approach in advance, casting Greater Invisibility, then Mind Blasting them all in a concentrated group before grappling/Misty Stepping to a place where it can eat the brains in peace? Is that the gist of it? It could work, although it would work a lot better if the Mind Flayer had an Intellect Devourer along to detect the Shadow Monk sneaking around. That would raise the difficulty to 30% over Deadly of course, but it could work.
Yes that's the general idea.
If the mind flayer is unlucky and goes last, it can misty step into position and mind blast.
Greater Invisibility is so it can attack with advantage and then grapple without the players stopping it. A lot of character abilities like cutting words etc require sight to work. It also makes it a lot harder to hit
Since it's an arcanist, it wouldn't be unheard of for it to have placed a few alarm spells around the place.
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