The Andromeda Key (アンドロメダのカギ Andoromeda no Kagi) is an object from the Mega Man Star Force series that was under the possession of Omega-Xis in the first Mega Man Star Force game. In the game, it looks like a glowing key with a red gem. In the Mega Man Star Force anime, the Andromeda Key is an orb with a claw on it. It is required to awaken Andromeda.
The Andromeda Key was taken by Omega-Xis after he betrayed Planet FM, and escaped to Earth with it. The other FM-ians followed after him and attempted to retrieve it from him. Despite Geo Stelar asking about it, Omega-Xis never attempted to answer it completely, saying it has nothing to do with Geo.
Eventually it was revealed that the Andromeda Key was intended to awaken Andromeda and use the weapon to destroy the entire planet by having Andromeda devour everything. When confronting Queen Ophiuca, she revealed that the Andromeda Key was simply one of two parts, one being the key that awakened the beast, and the other being the controller that tamed the beast. Ophiuca attempted to convince Omega-Xis to join her and give her the Key, and she could coerce the FM King Cepheus to give her the controller, thus allowing her and Omega-Xis to rule Planet FM together.
When Pat attempted to form a BrotherBand with Geo, his alter ego, Rey, took control of him, with the intention of using the BrotherBand to get the Andromeda Key from him, but Omega-Xis caught on before it was too late. Later, Pat and Rey became Gemini Spark and Mega Man had to fight him, eventually defeating him. The betrayal that Pat had committed and Pat not stopping despite Geo's pleas, left Geo in a state of depression, making him lose the will to fight.
Due to that, Omega-Xis decided to leave Geo as he would be a liability, but due to that, the revived FM-ians managed to ambush him and defeat him, even though Harp Note attempted to help out, as she was defeated as well. From this, the FM-ians claimed the Andromeda Key from Omega-Xis and gave it to the FM King, who used it to awaken Andromeda.
Nothing else is known of the Andromeda Key, as after Mega Man confronted Andromeda and destroyed it, the key was rendered useless.
The Andromeda Key served the same plotpoint as it did in the game, though its uses were much more different. It was first used by Omega-Xis to destroy a large asteroid as a cover to escape, as Omega-Xis hid it inside his own body. Cygnus and the other FM-ians attempted to take it back, but were pushed back by Mega Man, though whenever Geo asked Omega-Xis about it, the latter simply said that it had nothing to do with Geo, similar to the game.
However, after Cygnus Wing returned and made the other FM-ians, save for Gemini, work for him in exchange for giving them the ability to EM Wave Change without a human host, they managed to overwhelm Mega Man, defeating him and separating the two. It was retrieved by Cygnus Wing when Omega-Xis revealed the location, due to Cygnus Wing threatening to kill Geo, who was rendered unconscious from being defeated by the other FM-ians. After taking it, Cygnus Wing used it to summon Andromeda, and planned to destroy the Earth, along with the other FM-ians, so that Cygnus could receive all the credit for himself from Cepheus. (episode 29)
When Mega Man returned and faced the FM-ians once more, this time with the aid of the Star Break, Gemini Spark came and wounded Cygnus Wing with their Gemini Thunder from behind in the ensuing confusion, so that they could take the Andromeda Key. Gemini Spark revealed to Cygnus that they intended to use Andromeda to conquer Planet FM and overthrow Cepheus, much to Cygnus' horror. When Mega Man defeated the other FM-ians, he used the Atomic Blazer attack as Fire Leo to defeat the leftover FM-ians, Gemini Spark, and Cygnus Wing. The latter ended up losing his wave form, separating into Cygnus and Tom Dubius, and Cygnus ended up being destroyed. After that, Mega Man took the Andromeda Key and damaged it into expelling all the negative energy from it, thus shutting down Andromeda. However, Pat returned to the scene and found the Andromeda Key, though now it was powerless and couldn't be used.
After repairing the key, Gemini Spark then made a duplicate and gave that to the other FM-ians. He told Cancer that the Key can be refilled with the negative emotions of humans, which instigated the FM-ians a series of attacks for awhile, while Gemini Spark would try to draw the negative energy using the real key. However, the humans negative emotions were too weak to refill the Andromeda Key at all, causing Gemini Spark to slowly get impatient. After Cepheus ordered the FM-ians to return home, Gemini Spark took drastic measure to refill the key. He went on and slaughtered Taurus, Wolf, Ophiuca, Libra, and Crown, and absorbed all their negative emotions, which produced a much greater quantity than humans. It wasn't fully charged, as they failed to get one last sacrifice for the key, but Gemini Spark knew that there was enough to reactivate Andromeda. After Andromeda was destroyed, or more critically damaged, from the AM sages' kamikaze attack, Gemini Spark White decided to destroy his Black counterpart, thus completing the Andromeda Key.
The completed Key then healed Andromeda to the point that it was never damaged to begin with. At full power, Andromeda drills into the planet's core and releases a large amount of negative energy, which would cause the planet's core to self-destruct. Mega Man knew the only way to stop Andromeda was to get the Key, which Gemini White had, and he left the planet to escape it's imminent destruction. Mega Man catches Gemini White, but he throws the Key back to Earth, resulting in Mega Man chasing after it. Mega Man grabs it with Omega-Xis and shatters it completely, permanently destroying the Andromeda Key, therefore rendering Andromeda permanently disabled.
While in the games, the Andromeda Key was simply a device used to awaken Andromeda, the anime had more functions with it.
The Andromeda Key was able to activate Andromeda and control it, whoever holding the key being the one that could give it commands. The key is powered by negative energy, coming from negative emotions such as anger, fear, sadness, and so forth, with the appearance seeming to resemble a galaxy within. Despite being damaged, after it was repaired, the Key was capable of absorbing the negative emotions from others and convert it into energy.
However, human emotions proved too weak to be utilized, but when FM-ians were destroyed, and the Key absorbed their negative emotions, the Key showed itself to be able to obtain much more energy, appearing in a sort of liquid form within the container. From the destruction of five FM-ians, the Key was able to fill up to near completion, and in need of a single more sacrifice to be fully operational. However, despite not having that final sacrifice, the Key was still able to reactivate Andromeda to be used.
When Andromeda was critically damaged, and seemed to be completely destroyed, Gemini Spark White destroyed his Black counterpart, and absorbed the energy to completion, it showed that so long as the Key was fully restored, Andromeda would always regenerate. That said, if the Andromeda Key was destroyed completely, Andromeda would completely cease to function.