A NetHack amulet
that prevents your body from changing form. Specifically, polymorph
effects will not work. The amulet also confers protection against being slimed
by the feared green slime
-- this effect alone makes it worth wearing if you don't have an amulet of life saving
or an amulet of reflection
Eating the amulet has a small chance of bestowing permanent unchanging as an intrinsic; this can be a mixed blessing. Otherwise, there is no useful effect, though doing so may return you to human (or elven, dwarven, whatever) form.