Elysium

Elysium, known in Japan as Heaven (ヘブン), is a "perfect world" which orbits high over Terra (Earth in Japan). The Master lived on Elysium with Mega Man Trigger, Sera, the Mother Unit in charge of Elysium, and other units. However, The Master, after going down to Terra with Mega Man Trigger and seeing the people there living happily, saw that it was better than living forever on the perfect, yet cold world of Elysium, and asks Mega Man Trigger to end the Master System of Elysium. Elysium also houses the Carbon Reinitialization Program.

History
The ancient humans who created and lived on Elysium were not subject to aging, sickness or hunger thanks to their advanced technology. Nevertheless, at some point all of the original humans died or disappeared due to unknown circumstances, save for the Master. The Master himself lived on Elysium for 3,000 years before eventually returning to Terra and perishing himself. The Library on Elysium contains copies of their genetic information.

The systems on Elysium were maintained by Sera and Yuna, who together made up Elysium's mother computer. Other android units were created, including Bureaucratic Units like Mega Man Juno, and Purifier Units like Mega Man Trigger (aka Maverick Hunters).

Elysium is essentially a space colony that orbits Terra. It's completely spherical except for a single, horn-like protrusion. It is apparently big enough that it can be seen from Terra like a moon. There is some speculation that Elysium is actually the moon, but MegaMan's memory of the Master shows the actual moon in the Terran sky.

In Rockman DASH: 5tsu no Shima no Daibouken!, Mega Man and Roll discover an enormous tower that has risen out of the sea. When MegaMan reaches the top, he finds a broken elevator terminal with the obscured term "HEAVEN" written in ancient text. Additionally, Mega Man infers the damaged top of the tower indicates it may have once been even taller. This implies the tower may have once served as an orbital elevator for transport to Elysium and most likely destroyed, severing the link between Terra and Elysium directly.

Facilities

 * Shuttle Bay - A hangar where shuttle crafts can launch from and dock in Elysium.
 * Defense Area - A labyrinth-like zone connecting the Shuttle Bay to other areas, seemingly to keep out aggressors. Many strong Reaverbots inhabit this area. Also, in some parts the local gravity can be adjusted.
 * Side Area - Also known as the residential zone. This is where ancients lived, including the Master. The advanced technology gives the illusion they are on individual islands out in an endless sea, and the various islands are connected by warp gates. An elevator in the center island connects to the Shuttle Bay.
 * Mother Area - The area set aside for Sera and Yuna, Elysium's mother computer. It's guarded with numerous powerful, boss-class Reaverbots.
 * Library - The area where the genetic information of the ancients is stored. The Master originally ordered MegaMan to destroy the Library, which caused his betrayal of the system. Sera waits here to fight MegaMan.

Notes and trivia

 * In the ending to Mega Man X5 where X doesn't remember Zero, X claims his dream is creating a place called Elysium where humans and Reploids can live in harmony. This is believed to clarify the X series' connection to the Legends series (in the Japanese games, X refers to it as Heaven), although presently there's no sign that X was involved with the Legends series' Elysium in any way.
 * Elysium was most likely the original name for Neo Arcadia.
 * The symbol in the top of Elysium is the same in the Library door and in the center of the Library floor, suggesting they are related to the Carbon Reinitialization Program. The symbol is also present in Sera's second form.