According to the D&D 5e Monster Manual, regarding destroying a phylactery, it says:
Destroying a lich's phylactery is no easy task and often requires a special ritual, item, or weapon. Every phylactery is unique, and discovering the key to its destruction is a quest in and of itself.
— p. 203, Death and Restoration
However, no further information is given on how this is typically done, or what is involved specifically.
I get that this is meant as a plot hook for the DM, and that the DM is meant to fill in the blanks as befits their story/campaign, and that the intention here is that different lich's phylacteries must be destroyed in different ways, rather than a one-size-fits-all method for destroying any phylactery.
However, it would be easier for me to come up with something if I had some examples to work with from existing adventures or additional lore on liches not included in the D&D 5e Monster Manual. Is there anything published in any edition of D&D that describes how to destroy a (specific) lich's phylactery?
Best Answer
Yes, several.
Most sourcebooks prior to D&D 5th edition make no mention of the invulnerability property of a phylactery, implying that they can be destroyed normally. Most lich phylacteries in D&D 3rd edition for example are merely extremely tough, and can be destroyed with sufficient force.
However, special methods for destroying a lich's phylactery in various editions of D&D include:
Additionally, you may find it useful to research the method of destruction of artifacts, which have historically required unique methods of destruction. See the 5e Dungeon Master's Guide, p. 221, for examples.