The Mnemic Egg is an extremely powerful Argonian artifact that is the physical link between the Hist and the Argonians, which also acts as a gateway into the collective mind of the Hist trees. When the Aldmeri Dominion invaded Black Marsh in 2E 582, they sought out the egg to use against the Ebonheart Pact. By using the egg, they were able to tap into the immense power of the Hist and sever the Argonians' connection to it. It could also stop Argonian eggs from hatching, which could drive the race to extinction.
The Dominion was very close to succeeding, until the Vestige, their allies and the Vicecanons stormed the Dominion's base, an Ayleid Ruin called Loriasel and killed Ruuvitar, who possessed and corrupted the egg. The Vestige then entered the dreams of the Hist through the egg and destroyed the corruption within.