How do i replay/farm Xesht?

It's unclear to me if i can do it right after the 1st encounter, and if so, how, or if i have to travel on the atlas until i find a new "breach zone" and only then i can do all the process again (kill the hives, etc) in order to face it a 2nd time.
Anyone can help?

Trying to figure this out for Ritual too.
Can't allocate a load of points till I beat the boss, can't figure out how to actually get to the boss (a second time) because there's no Realmgate on my Atlas.

Is there another way we are supposed to rerun bosses ?
Or does the Realmgate appear later on.