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Meant to reply to this earlier. Had the tab open!

Stored Aspects will be moved to the Codex of Power for your Eternal character seamlessly when the season ends. It will take the highest of each Aspect that you may have collected into the new Codex.

So, let’s say you have an Aspect in your stash on your Eternal character and Season 4 begins, the Aspect will be automatically added to your codex and Aspect item will be destroyed.

But let’s say you had a max roll of that same Aspect from your season 4 character’s stash, it will then seamlessly replace your lower roll Aspect in Eternal and forever be in your Eternal codex that way.

Just a small caveat is that this happens automatically for anything in your account stash when the season transition occurs. If you have them in a specific alt character’s inventory, you will need to just hop on them after the Season rollover and those will then seamlessly move to the Codex as well.

The seasonal codex does reset. The eternal one is forever.