The Eldritch Decay is the final Atlas quest, given to the player by Zana in the storyline's epilogue.
Defeat the Uber Elder.
To spawn the Elder at the Shaper's Realm, have the Elder influence spread next to a Tier 16 map and defeat the Guardian to turn it into an Elder-influenced area. Once that's done, defeat the other Guardians to spread Elder influence to those maps. As long as all four Tier 16 maps are Elder-influenced, the Elder will spawn in the Shaper's Realm. Then, defeat the Elder Guardians and finally the Elder.