Nothing in D&D is truly indestructible. However, the more powerful the Lich the harder they can make finding and destroying the phylactery.
Powerful magical items are an excellent choice, as the party will take the phylactery thinking that it is treasure, and then quickly find themselves battling the lich again and again until they figure out what it is.
Artifacts can work if he lich is particularly powerful and old. It is a bonus if the artifact actually has a history with the party member's home kingdom or their allies.
I was in one campaign where the Lich's phylactery was located in his deity's throne room. That makes it extremely difficult to obtain, and permanently killing the lich would be an epic campaign on its own.
Powerful magical items are an excellent choice, as the party will take the phylactery thinking that it is treasure, and then quickly find themselves battling the lich again and again until they figure out what it is.
Artifacts can work if he lich is particularly powerful and old. It is a bonus if the artifact actually has a history with the party member's home kingdom or their allies.
I was in one campaign where the Lich's phylactery was located in his deity's throne room. That makes it extremely difficult to obtain, and permanently killing the lich would be an epic campaign on its own.