I'm planning a lich as a BBEG villain in an upcoming adventure. I want it to be possible for the party to find the phylactery before facing the lich, as well as discovering how to destroy it.
If they manage to learn where it is and how to destroy it, and do so before facing the lich, would destroying the phylactery also destroy the lich outright, or would it simply prevent rejuvenation (meaning when they do go fight the lich, then assuming they manage to slay it, it stays dead for good)?
The only thing I've found on this site (and I don't trust other sites) is this answer to a question about liches and phylacteries, which says:
The lich does not die immediately, but it is then without a phylactery. This means the lich will not rejuvenate.
However, there's no citation for this. The lich entry in the Monster Manual doesn't seem to describe what happens in this situation (meaning, does the lich die instantly or not), so is there any other information about liches (ideally D&D 5e, but previous editions of D&D are acceptable as well) that describes what happens in this scenario?
Best Answer
It keeps Existing
In 5e, things do what they say. A phylactery is required for a lich's Rejuvenation ability and no other ability, including the Undead type.
So by inference, "If it has a phylactery" means that it is possible for the creature to not have a phylactery. Proof that it is possible for the lich to exist without a phylactery (up until they're destroyed and cannot rejuvenate).
As an analogy think of a cellular phone. A cell phone can operate for a time independent of other physical objects, but requires a charger to renew its abilities. If the phone charger is destroyed, does the cell phone immediately stop working? Or does that destruction merely prevent renewal?
Is it possible that a particular lich can be destroyed by destroying the associated phylactery? Yes, but that individual lich would have a glaring weakness that others do not have.