A conversation between Shadow and Sensei discloses that when Shadow opened the Gates of Shadows, it not only caused him to lose his body and obtain his shadowy appearance but also acquired a special power. The power, Sensei states, is that Shadow has the ability to cause an Eclipse on the sun if he desires to. This Eclipse will unleash his foes' darkest side, therefore making his opponents stronger, but will also allow Shadow to win precious rewards. These rewards are the Shadow Orbs. Sensei also says that Shadow can call off the Eclipse any time he wishes to. Sensei then proceeds to show Shadow how to utilize this power.
The game map denotes the Sun in the top right corner. It stays fixed there in any part of the map. This Sun is the switch to turn Eclipse on and off. Clicking it once will turn the Sun's color from yellow to black and clicking it again will make it turn back to normal. This can be done as and when willed. Activating Eclipse will increase the difficulties of all modes of play (with the exception of Challenger fights, raids, and Ascension).
When the Eclipse is active, all Tournaments, Challenges, Survival, Duels, and Storyline Fights will reward a considerable amount of Green and Red Shadow Orbs (in addition to the coins rewards). Additionally, beating a demon boss when the Eclipse is active will grant the player with additional purple orbs. As a consequence, the enemies will become stronger, in the sense that their offense and defense will be increased. The strength boost varies depending on the enemies; Tournaments, Challenges, and Survival fighters will receives minor to moderate boost. Bodyguards (especially Assassin and Master) will receives a major strength boost. Demon bosses and mini-bosses will receive a minor boost.
There is also a feature known as Eclipse Replays. When all the Tournaments, Challenges, and Storyline Fights in an act have been completed, they are available to be replayed by activating the Eclipse. Beating any of these replays will reward a minuscule amounts of green Shadow Orbs, Eclipse-related Achievements, and Boss Weapons which are obtained by defeating the respective boss. The Eclipse Replays are rendered temporarily unaccessible during the Interlude, as it alters the Storyline Fights, Tournaments, and Challenges. With the Eclipse Replays being unavailable, the boss weapons become temporarily unobtainable as well. After defeating a boss in the Interlude, the Eclipse Replays will return and the players can obtain the boss weapons by beating them in Eclipse Replays (if they do not have the weapon yet). The Tournament and Challenge Replays will also return after completing their Interlude counterparts.
Shadow Orbs are of three types: Green, Red, and Purple. Green orbs can be obtained in almost every fight, in variable amounts. Red orbs, however, are restricted to Storyline, Tournament and Challenge fights. If played while the Eclipse is active, these fights will reward the player with greater amounts of orbs.
Purple Shadow Orbs are the rarest and hardest to obtain. They can be acquired by winning a demon fight when the Eclipse is active for the first time. They are also available in the wheel of prizes in Ascension.