Donsilan
In Innocence Seekers and Yume ni Aruku, the Donsilans (ドーンシラン, Nodaimese: dōnsilan, Namari: どーしゃ dōsha, Trulauian: dūsil) are an alien race which inhabited the galaxy about two million years ago. No one knows what exactly happened to them or when they disappeared; all that is known is that they suddenly disappeared from the galaxy at some point in time, leaving behind various artifacts possessing immense magic power.
In terms of appearance, Donsilans resemble humans. They, like humans, are capable of using magic, and much of their technology revolved around magic. However, they are far more powerful than humans, with a single Donsilan able to match even a powerful demon in battle. Hence whatever memory humans have of Donsilans have been conflated with their religion, often imagining Donsilans as gods or something similar. Likewise, their technology is unmatched; Donsilan technology has so far evaded all attempts at reverse engineering.
Innocence Seekers
In Innocence Seekers, the Donsilans are noted by many characters to have had a profound impact on the galaxy. They know full well that the course of history has changed whenever someone finds a Donsilan artifact and uses it. As Donsilan artifacts are found all over the galaxy, especially the four "main" planets of Earth, Ocypsa, Fuenod and Elrodom, as well as the fact that one of these artifacts can potentially wipe out a significant chunk of a planet's biosphere, the Zodiac Warriors and Puella see it as a priority to search for and secure any Donsilan artifact they hear about.
It is hypothesised by a few people that the original Donsilan homeworld is Earth. However, there is very little evidence that this is the case; if it were, then one would expect evidence to be visible in the geological record. Instead, the dominant theory is that they originated from a world which is, in many respects, very similar to Earth. However, it is known that they lived in conditions very similar to that of humans, and the vast amount of planets which are seemingly human-habitable is often considered evidence that the Donsilans engaged in extensive terraforming (in fact, the World Orbs found on many inhabited and uninhabited planets, including Earth, may have been Donsilan terraforming devices).
Innocence Seekers: The Black Rose
WARNING: SPOILERS AHEAD
In Innocence Seekers: The Black Rose, the Donsilans were thought to have died out. However, that was proven untrue in Innocence Seekers: Akari of the Light, when a solitary Donsilan appeared about 100 kilometres west of Tokyo. This Donsilan was hostile, but claimed that she was not working for Gakran. As the Donsilan was hostile, Amelia, at the time the galaxy's strongest magical girl, set off to confront her, taking Avala, the witch who had formed a contract with Amelia two years ago, with her. Amelia confronted the Donsilan and eventually a battle commenced. However, even when unisoned with Avala, Amelia was forced onto the back foot in the battle. The Donsilan was eventually defeated when Erika and her contracted witch Taranna arrived and helped Amelia defeat the Donsilan.
A second Donsilan appeared very late in Akari of the Light, this time directly confronting Akari and her group. The ensuing battle, however, was very short, after Mirai's Inherent Ability, Overwhelming Blow, activated due to sheer luck.
In Innocence Seekers: April Light, a small army of Donsilans was spotted about 150 kilometres south-by-southwest of Yaezora. Knowing full well that sending anyone to confront the army would mean certain death, Erika ordered the spaceship NSNS Himiko to bombard the Donsilan army from orbit.
It is later explained that these sporadic appearances are because the Milky Way Galaxy was not the only galaxy Donsilans inhabited. The Donsilans had intended to return to the galaxy to reclaim what they thought was rightfully theirs after their civilisation in the galaxy collapsed due to what they say was a "powerful demonic being", and did not anticipate humans leaving Earth (via Portal Monoliths) and inhabiting the galaxy. After Lily explained the situation to the Donsilans, they left, saying that they do not intend to fight humanity for the right to live in the galaxy. However, they told Lily and Jacinta that they can be called upon should the Zodiac Warriors or Puella request their help.
END OF SPOILERS