The party, going up against a lich, finds out the location of its phylactery and how to destroy it. They get to its phylactery without the lich knowing about it and manage to destroy the phylactery.
Would the lich be aware that its phylactery had been destroyed? Is there anything in any official D&D publication that states whether or not a lich would know that its phylactery had been destroyed? I'm playing 5e, but lore from any previous edition is welcome if 5e doesn't have any information on this.
Best Answer
Possibly immediately, but at the very least he will figure it out sooner or later, how sooner or later is up to the DM.
The Monster Manual entry for liches says:
So at the very least a lich will begin to notice the degradation of his corporeal form.
There is evidence, however, that a lich may immediately know that its phylactery has been destroyed. In Waterdeep: Dungeon of the Mad Mage, the party can encounter a lich; we see this statement about it:
It is not spelled out how the lich might know the party destroyed the phylactery, but the lich seems to know it was destroyed when the party encounters it.
Further details, W:DotMM spoilers ahead.