Where to use Star Key?

[HMetagross]

Found the key after killing the big red slime but not sure where to use it

[Asael]

It's for the cathedral door close to where you came out to the surface.

[gk00003]

demo: how to use the star key in the star key door.

[SE@L]

Chapel Door in the lower Bastion area.
in Lord Belricks chambers.
And I believe it may also open the rune door that acts as a teleport.