My review of the other three items:
Navel of The Moon. Quite neat, and perhaps the only actually useful item of the four

I would totally allow it to upgrade a magic item - not only that it can turn a non-magical item into a +1 item, but that it can turn a +1 item into a +2 item etc.
Skull Chalice of Chgakare. Quite useful. For that bratty teen princess, that is.

For everyone else, a bit confusing (because it seems to talk about two kinds of charges) and a let-down (since the royal power is several magnitudes more awesome than the ones grubby 'venturers get). Though I guess this is Mwaxanare's only hope (that they throw it to her in disgust

) ...
Black Opal Crown. Oh em gee.

This must be the coolest item ever.
There's a bit of mechanics, but it all pales in comparison to the last sentence. Essentially, the crown allows its attuned wearer to replace a light source within line of sight by darkness. Any light source come in mind?
Well, this the designer thought through. The whammy comes last: "If Dendar the Night Serpent is attuned to the crown,
and targets the Sun, it is replaced by a Sphere of Annihilation."
Just like that. A sun-sized Sphere of Annihilation. Any campaign that ends like that must be the
definition of awesome.