A genius even by Ancient standards, Ikaros was a pupil of Janus’, who collaborated with his mentor to develop Charybdis, an experiment that they believed would allow them to avert the evolution of the iratus bug, thus erasing the Wraith from existence.
Although Janus and Ikaros were denied permission to complete the experiment due to the risk of them creating a timeline where Atlantis would never even exist, Ikaros became obsessed with perfecting the experiment after his parents and sisters were killed in a culling, to the point that even Janus believed it would be dangerous to allow him to continue. To achieve his goal, Ikaros downloaded his mind into a quantum computer that he had built himself to grant himself the necessary processing power to ‘do the math’ and calculate the numbers necessary to create a reality that would erase the bug while leaving Atlantis intact, but he was disconnected from the city mainframe and left in a lab for the next ten thousand years until he was discovered again by the Atlantis expedition.
Although the expedition agreed to aid him in finishing Charybdis, the experiment failed to function properly, creating multiple alternate realities where the Wraith no longer existed, each one in varying states of dystopia ranging from the Ancients having lost most of their technological knowledge after advancing too far, to them having become conquerors at least as bad as the Goa'uld without the Wraith to either inspire them on to new heights or keep them in check to prevent them from becoming as bad as their enemies.
Fortunately, the six members of the expedition present when Charybdis was activated- John Sheppard, Elizabeth Weir, Rodney McKay, Teyla Emmagan, Ronon Dex and Radek Zelenka- managed to survive the process, although each of them was transferred into other realities created by Charybdis, time being so twisted in the process that even their ages were altered, Teyla and Zelenka gaining thirty years while McKay was regressed to a teenager. At the same time, McKay was close enough to the computer containing Ikaros when Charybdis was activated for Ikaros to transfer most of himself into McKay's subconscious, although he was forced to remain dormant in McKay's mind, as McKay had lost his memories and regressed to a teenage version of himself in a reality where Ikaros was essentially regarded as a devil for his role in causing society in this world to collapse, until McKay discovered the remains of this version of Atlantis and regained some of his old memories. The two attempted to assemble some Ancient technology from Atlantis to get the Stargate working again, but McKay was captured and sentenced to death as a heretic.
While he and McKay were being held prisoner and facing execution, Ikaros suggested that they Ascend in order to survive- reasoning that Charybdis wouldn't keep trying to kill them if they couldn't interfere with its efforts to sustain itself-, but McKay rejected the idea as he preferred dying a pointless death to living a pointless life watching his friends cease to exist, challenging Ikaros by pointing out that he had sought to save lives himself. Shortly before their execution, McKay and Ikaros were rescued by Ronon and Teyla; exploiting the fact that the Stargate network was trying to draw people to the originals needed to restore reality by seeking out their specific quantum states, Ronon had used the ear of a deceased alternate McKay to ‘home in’ on the original.
Using the skull of the John Sheppard of the world they were currently in to track down the original- who had already found the original Zelenka and Elizabeth in a world where the Ancients had reached a point of stagnation, unable to develop further due to a lack of incentive to do so-, Ikaros was able to convince the panicked Ancients of this dying world to listen to him by partially Ascending to demonstrate what they should be capable of in the true timeline. With time running out, Ikaros and the six expedition members travelled through the Stargate once again, Ikaros's presence taking them back to their original reality in the moments before Charybdis was originally activated, allowing them to ‘merge’ with their past/alternate selves and avert the activation of Charybdis at the last minute. Having merged with the remnants of himself in the quantum computer, Ikaros completed his ascension, healing the last of McKay and Teyla's injuries before he departed. Later research in Atlantis's database revealed that, in this new reality, it had been Ikaros who first taught the other Ancients how to Ascend, showing that he had gone on to make a good life for himself in the absence of Charybdis. (SGA: "Mirror Mirror")