It is not necessary to defeat the Godskin Noble in order to advance in the Elden Ring. This boss is therefore optional.
The Godskin Noble appears in a ghostly form during a separate boss fight, which is one of the places where it might be encountered.
In Spiritcaller's Cave, the Godskin Noble in spectral form is the first opponent you will encounter during the boss battle with the Spirit-Caller Snail.
The Godskin Noble can be encountered in its corporeal form during the second confrontation in the Temple of Eiglay in Volcano Manor.