It looks like The Flash season 4 is going a very different direction from its predecessors, and the newest addition to the show will certainly have some scratching their heads.
That new addition is the DC villain Kilg%re, who will be one of the new villains Team Flash encounters this season. Kilg%re isn't really a person though, as the entity is actually a virus that can take control of anything electrical in nature. That means computer systems and power grids are susceptible to it, and that will cause plenty of issues for Flash this season.
The character's first appearance in Flash #3 had it taking over the world's computer systems, and even almost managed to transfer itself into a human brain in case the system faltered. The Flash was able to track it down before it could take over the clone, but after its defeat, Kil%gore would still show up from time to time, including a memorable run in Justice League International.
It was revealed through the course of that story that Kilg%ore had taken control of Maxwell Lord's mind, using his influence to increase Kilg%re's power. It was eventually defeated again, but not forever.
Most recently it appeared in issues of Cyborg, where Kilg%re's form received a massive and more threatening overhaul. It has also been shown in more human-looking forms over the years, so it is unknown what version The Flash will decide to use.
Flash showrunner Todd Helbing told CBR that the character is "a lot of fun," and should provide a nice change of pace from the typical evil speedsters villain that the show has used so much over the course of 3 seasons. Kilg%ore will also likely be linked to season 4's main villain the Thinker, who probably has some hand in creating it or bringing it to Earth.
The Flash returns on Tuesday, October 10th.