You can get it on Narukami Island, Tenshukaku domain at level 70, this material is obtained after facing Sijourna, it also transforms from Molten Moment and Ashen Heart using Dream Solvent on the alchemy table.
On Narukami Tenshukaku Island you can find Raiden Shogun, and to access it you must play Archon Quests. Being in Chapter 2 of Archon Quest, where in Act III. Omnipresence over mortals, you will face Signora to obtain this material.
This is a repeatable event that players will be able to do every week to earn Hellfire Butterfly drops.
Keep in mind that the reward will cost 30 resins for the first 3 weekly fights, and then 60 resins for each fight.
After introducing you this guide on where to find the hellfire butterfly in Genshin Impact, we hope you can reach the required level and find the eternal butterfly on the island.