How do you unlock the Erissalis encounter?

The gate here is a crafting bench, not a map marker. No amount of exploring turns Erissalis up on its own, and there is no separate chapter to clear first. You work toward the legendary Anipod, and the fight comes to you.

What appears once you craft one is an exclusive story, handed to you as a quest, with the encounter placed on the map. So it is not a wandering boss you stumble into at the right level. It sits in a fixed spot and waits, and it is a scripted one-off attached to that craft rather than something that respawns for farming.

The practical read: if Erissalis is what you want out of a run, spend your time on legendary Anipod materials rather than sweeping the region for a spawn. Everything else about the fight is downstream of that single craft.

Not settled: The name shows up as both Erissalis and Irisalis in the beta, and which spelling is the final one is not settled yet.