If I were a lich…
I would have a super-cryptic and yet painfully precise map that leads to where my phylactery is hidden – a “final challenge” to those who have defeated me. But you’d need to figure out all the clues and codes to get the map to lead you there. But really, the clues are purposefully obtuse and can be interpreted in multiple ways, and yet are exceedingly and needlessly precise (47 feet north-north-west of the shattered stone, without giving any way to find the shattered stone, for instance).
Of course the map itself is my phylactery. I mean that’s obvious if you know me… But who is going to destroy the only clue to finding an enemy lich’s phylactery?
To keep the phylactery from being accidentally destroyed, it isn’t going to be on PAPER of all things. It would be something like a bronze plate with hinges on all four sides so you can flip various “overlays” onto the map to “figure out” the clues. And on the back of the map there is of course a poem that only causes additional confusion as to how the whole thing works and where it leads.