How to Upgrade Legendaries to Rank 7 in Patch 9.2: Eternity's End

wowhead.com:

Patch 9.2 is getting closer to a release date, and along with the new activities, locations, and class changes, players will be able to upgrade their legendaries to Rank 7. We've been busy looking at the PTR to find out exactly what players will need to upgrade their legendaries, from the new Cosmic Flux currency to crafting higher item level bases using Progenitor Essentia and Vestige of the Eternal.

Legendary items can be upgraded to item level 291 at Rank 7 in Patch 9.2, although they require slightly different materials than previous upgrades. To go from Rank 6 to Rank 7, you'll need:

Upgrading from a Rank 6 to Rank 7 Legendary requires only 2,000 Cosmic Flux, instead of Soul Cinders or Soul Ash. Unlike the previously used reagents, Cosmic Flux can be obtained by participating in a myriad of activities such as participating in Zereth Mortis, Mythic+, Arenas or other content. We have a preliminary list to Cosmic Flux costs on a news post, but this is not a final list Cosmic Flux Acquistion

Rank 7 Base Items are created the same way as other base items, along with a Vestige of the Eternal optional reagent, which increases a crafted base by 3 ranks — crafting a Rank 3 base with the optional reagent results in Rank 6, and crafting a Rank 4 base with the optional reagent creates a Rank 7.Vestige of the Eternal are also crafted. Tome of the Eternal, the item to teach the Vestige of the Eternal, is awarded during the 9.2 Campaign, at the end of Chapter 5, Crown of Wills. This item is different than the similar item Tome of Origins from Patch 9.1, and in order to learn the