Quote Originally Posted by Vorpalchicken View Post
The demilich is specifically immune to damage from "necrotic, poison, psychic; bludgeoning, piercing, and slashing from nonmagical attacks" but resistant to "bludgeoning, piercing, and slashing from magic weapons"
So RAW that's no damage. And a good reason to not always follow RAW
Except that the monk's "unarmed strikes count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage."

The monk's unarmed strikes in consequences bypass the immunity, but since "resistance from magical weapons" explicitly *isn't* "overcoming resistance [...] to nonmagical attacks and damage", the Demilich's resistance doesn't apply either.

In other words by RAW the monk's strikes do the full damage.

There are many reasons to not follow RAW (chief among them being "I prefer my rule"), but this isn't one of them.