The Veil shattered like the flimsiest glass.
—Canticle of Silence 2:7”
Magisters Sidereal are the seven ancient members of the Magisterium, each a high priest to the Old Gods, who breached the Fade physically and entered the Golden City in -395 Ancient. According to the Chantry, through their actions they committed the Second Sin and caused the First Blight, releasing darkspawn upon Thedas.
The seven Magisters often plotted against their enemies, as well as one another, and to protect themselves, they used titles rather than their real names. Most, if not all, records of "The Seven" have been destroyed, including their names.
The Old Gods began whispering to their high priests from the Fade. Dumat was the first of them to address his servant. The high priest to the God of Silence was Sethius Amladaris, also known as Corypheus or The Conductor, and it was he who put the plan to breach the gates of the Golden City in motion. Dumat tempted Corypheus to visit him in the Golden City so that he can bestow godhood onto Corypheus, for Corypheus was said to be the most powerful and wise of the Sidereal, and a favourite of Dumat. The Conductor became obsessed with what he had heard. He shared the word of his god with other priests and acolytes of the Silence and they too became consumed with fulfilling the will of the gods. The first acolyte advised Corypheus that even with all their might they will not be able to reach the Golden Heart of the Fade and suggested an alliance with the high priest of Urthemiel, the Architect of Works of Beauty.
And so The Conductor met in secret with The Architect to tell him of Dumat's words. The Architect found the idea of raising a mortal above the gods blasphemous and refused to help Corypheus. However, it did intrigue him and so he decided to speak about this with his own god, Urthemiel. The God of Beauty then spoke to him and encouraged Architect to come to him in the Golden City so that he can bestow the godhood upon the Architect instead and give him the designs to make Thedas into a paradise. And so the high priests of Dumat and Urthemiel decided to work together, each for his own agenda.
The power of The Conductor and The Architect were, however, still no match against the Golden City, so they approached the remaining five of the Sidereal: the Watchman of Lusacan, the Forgewright of Toth, the Appraiser of Andoral, the Augur of Razikale, and the Madman of Zazikel. All of them craved the power of which Corypheus spoke, and so after seeking counsel with each of their chosen deity, they united under the pretense of helping Corypheus, but in reality each wanted all the power for themselves.
A hundred acolytes were chosen to gather unimaginable amounts of lyrium and slaves beyond counting in preparation for the entering of the Golden City. The lyrium and the blood sacrifices made the Veil rip open in front of them and the high priests entered the realm of the spirits with pride unmatched.
The exact sequence of events that followed did not survive to this day, as both the Andrastian Chantry and the Imperial Chantry changed the verses of the Chant of Light times beyond counting. Some sources, like the Andrastian Chantry, claim that the hubris of the Sidereal tainted the Golden City, turning it Black, and as punishment the Maker, a true god as opposed to the false Old Gods, and the one true deity residing in the Golden City, cast them down; other sources state that the city was already tainted and black when the seven arrived, and, according to Corypheus, they found no one in the city, and instead the throne was empty.
In either instance, the Sidereal returned to the realm of the mortals disappointed and without the powers that were promised them. They did, however, return with the Taint inside of them that did give them some powers similar to those of the Archdemons, such as command of tainted creatures like darkspawn and Grey Wardens, shapeshifting, and possession. It was around the time of their return when the First Blight broke out.
According to the Canticle of Silence, upon the Magisters' return, the sky was burning. The contemporary Archon followed the path of the burning sky and reached the Magisters Sidereal. When confronted, they attacked him, but did not harm him. The Archon tried to use magic to cleanse the tainted land, but to no avail. He then called on the Spirits of the Fade to grant him powers enabling him to defeat The Seven. He used his magic to scatter the Sidereal to the corners of Thedas and returned to Minrathous to await The Blight. 
Dragon Age: Origins - Awakening
Dragon Age II
Dragon Age: Inquisition
Other Magisters Sidereal
According to Codex entry: A Different Darkspawn?, the dwarves encountered three tall darkspawn that meet the description of the Sidereal, arguing over the failure of capturing the powers of the Golden City. As the story goes, one of them was killed by the other, and then devoured.
A codex entry for the constellation Draconis hints that there might have been an eighth Old God who was stricken from historical record and if so, it's possible its high priest also joined the Magisters Sidereal.Even if others are currently alive, Corypheus made no contact with any of them. He does not know if any of them have even managed to leave the Black City and return safely, and considers them a lost cause.
- According to Dorian Pavus.
- Dragon Age: The World of Thedas, vol. 2, p. 54
- Codex entry: Corypheus
- As revealed in Corypheus's recorded thoughts at Shrine of Dumat.
- Dragon Age: The World of Thedas, vol. 2, p. 55
- Dragon Age: The World of Thedas, vol. 2, p. 57
- Canticle of Silence 3:1
- Canticle of Silence 3:16-3:19