Chaos dragon: a dragon with polymorphing breath, and whose scales grant unchanging. The dragon also has unchanging naturally (which raises the question of what happens if you polymorph into it, then die).

Artifact ring Nenya (Galadriel’s ring from Lord of the Rings): a mithril ring of warning that also confers unchanging. If possible, its warning should apply level-wide, not in a specific radius.

Unchanging prevents you from starting to turn to stone (it already prevents starting to turn to slime), and if you are in the process of turning to either, it halts the process but does not cure it.

When killed by a monster that brings you back as something else (zombie, vampire, green slime), you might not die; you may continue playing in that form with permanent intrinsic unchanging. This might make it actually beneficial to get killed by a vampire, so the chance of continuing can’t be 100%, or else it can only work in a limited set of mostly-uncontrollable circumstances (like maybe a vampire’s or zombie’s bite takes you down to exactly -1 health). Green slime death is probably okay to stay at a 100% chance, though. Also needs some thought for how the unchanging squares with being a vampire and wanting to #monster into different forms. Perhaps intrinsic unchanging, unlike extrinsic, allows a monster to change into its normal alternate forms, but nothing else. Some other downsides should possibly be present too, like you don’t get your normal amount of HP and are stuck with the amount given by your new form.