
Originally Posted by
Tarukaze_Mort
To expand on this;
One of the story arcs of Stormblood involved Krile being captured and taken to an imperial research laboratory, where a scientist named Aulus was able to study her and from that information artificially recreate the Echo. The artificial version of the Echo is called the Resonant, and as far as I know was given to two people: Fordola and Zenos.
If you then think all the way back to the Leviathan questline back in ARR patch 2.2, it's revealed there that one of the abilities the Echo can grant is that when your body dies, you can persist as an aetheric being long enough to possess another person (sahagin priest).
When Zenos killed himself at the end of Stormblood, his soul lived on and possessed one of the dead resistance soldiers (the disguise you refer to), while his body was buried. Elidibus later dug up the body and possessed it for his evil Ascian schemes, and so Zenos went on a new hunt to reclaim his body.
There's a cut scene somewhere in Shadowbringers (can't remember exactly when) where we see Zenos face off against Elidibus, and it cuts to black before we see the fight. There's then another scene later on that follows Gaius and Estinien in the Imperial Palace and they confront what they assume is Elidibus in Zenos' body, only to find out that it's actually Zenos in Zenos' body, who says Elidibus gave up the body rather than fight him.
There is a third known Resonant
Misija from the Save the Queen quests
As for the Zenos vs Elidibus cutscene, it's just before we activate the giant Talos, the quest where weare resting with the Exarc