After Louisoix (the old guy) teleported everyone away he was hit by Bahamut's megaflare but didn't die. The failed sealing ritual left a huge amount of aether around him and combined with the prayers of all the Eorzeans at the time it cause Louisoix to be turned into the primal Pheonix. Using that Aether Louisoix/Pheonix attacked Bahamut, defeating him but Bahamut managed to capture his spirit before he could disappate. The remaining functioning parts of Dalamud where designed to keep Bahamut in a constantly summoned state so he couldn't be resummoned so they began to restore him deep under the ground. However Bahamut at this point had tempered agents in the coils and so was less a prisoner and more of a master at this point. He is eventually beaten at the end of the Binding Coils when the Internment hulks keeping him summoned are switched off.
This video has the GCI scene of what happened immediately after the End of an Era cutscene. It will show the lead up that you have seen first though.
https://www.youtube.com/watch?v=1xOOFCltZuc
This event happens 5 years before the start of ARR. The Binding Coils story plays out after the defeat of the Ultima Weapon in the 2.0 story. Echo visions are usualy visions of the past.