The X-Event is an experiment started by XGaster, and is the reason for Underverse's main point of conflict. It is the event created by whoever has a half or whole of the Overwrite soul, one who wants to create something new in the X Tale universe or the multiverse.
It was originally a being with Frisk's body and Chara's soul. XGaster takes all of the ideas and creations of the X-Event and refine them on his own, using it only as a tool. Due to this awful treatment, X-Event tried to end its own life by forcing Cross to kill it, which would prevent XGaster from overwriting the universe. However, after finding out that the half of the other purple soul no longer belongs to XGaster, it gave everyone their memories from previous timelines back to rebel against XGaster and gain the completed Overwrite button. This plan ended up failing by Cross's intervention.
Cross destroyed Frisk's body and took the X-Event's soul, becoming the new X-Event. However, he could not overwrite, as his monster soul was not compatible with the purple soul's abilities. The X-Event's soul and its abilities were developed exclusively for a human vessel. Therefore, Cross could only use some of the human's abilities and the Hack Knife, which can corrupt codes from almost every universes.
Three identities have called themselves the X-Event, with them being