|Name in D'ni||sirus|
|Died||Leevobro 2, 9480 DE|
Sirrus (9432 DE – Leevobro 2, 9480 DE) was the youngest son of Atrus and Catherine, and brother to Achenar and Yeesha. He was most likely named after the star Sirius as a nod to Atrus' deep love of astronomy.
Sirrus was born on Myst, about two years after Achenar. When he was eight, his great-grandmother Ti'ana died. His father, lost in grief, buried himself in his experiments, neglecting his family for extended periods. Sirrus saw this as rejection, and eventually grew bitter toward his father, despite their shared interests. As Sirrus and Achenar grew older, Atrus began to take them along to his Ages. Unaware of his son's growing maladjustment, Atrus allowed Sirrus free reign to the Ages of Myst, and he seemed to take great pleasure in subjugating their native inhabitants, plundering the wealth of the Ages to live in luxury.
In 9641 DE, he and his brother devised a plan to rid themselves of their parents' oversight, and take more absolute control over the people in Atrus's Ages. They used the abandoned memory chamber on the Age of Serenia to construct a memory exchange chair, enabling them to swap consciousnesses with someone else. Their plan had been to "possess" their father, taking his place and acting out their demands under his guise. They tricked their mother Catherine into going to Riven and trapped Atrus in K'veer by taking one page from his Myst Linking Book. But their plan ultimately fell apart due to the brothers' substantial mistrust of each other. Sirrus ultimately abandoned the plan and traveled to Spire, one of the two books that Atrus placed in the Myst library to entrap any greedy interlopers who might uncover his lost Myst Linking Book.
Without a way to return to Myst, Sirrus found himself trapped on Spire for nearly twenty years. He spent years attempting to find his way to the ground in search of the Linking Book left behind when Atrus first visited the Age, but was ultimately foiled by the Age's unique lack of any solid planetary surface.
In 9475 DE, Atrus sought to re-establish contact with his estranged sons. Sirrus, still angry over his neglect in childhood and unable to empathize with his father, was furious to learn that he not only had a sister, but that Atrus was teaching her regestoy—something he ultimately refused to do for his sons. He devised a way to escape from his prison by using Spire's resonant crystals to destabilize the nara in the Age's new secure linking chamber, and kidnapped his sister, intending to swap consciousnesses with her and fool Atrus into teaching him regestoy. He was foiled by Achenar and the Stranger, who freed Yeesha from her brother's grasp in Serenia's Dream world. Untethered from his sister's mind, Sirrus's consciousness was lost, unable to return to his body. Effectively, he died.
In the game Myst, Sirrus and Achenar was not imprisoned on Prison Ages but in Trap Books, Sirrus in a red book and Achenar in a blue book. Sirrus is able to communicate through the linking panel and attempts to trick the player into bringing him all the red pages, scattered throughout the Ages of Myst, to complete his book. If he succeeds, the player is trapped in the book and Sirrus is set free in the Myst library. If the player frees Atrus instead, Atrus destroys the red and blue books, trapping Sirrus and Achenar in the void between Ages.