Benjamin "Ben" Kirby Tennyson, also known as Alien X, is the main antagonist of Alien X-Tinction, which is the finale of the 2016 reboot of the Ben 10 franchise. He is an alternate evil counterpart of Ben Tennyson.
He was voiced by Fred Tatasciore.
This Ben was normal as the reboot version of Ben Tennyson. However, Vilgax consistently attempts to steal the Omnitrix from Ben until a particular fight when Gwen and Max tries to assist him against his wishes. Tragically, Vilgax used his optic lasers to destroy the Rustbucket, killing Gwen and Max in the process.
Horrified and enraged by this, Ben used Four Arms to scatter the Omnitrix, causing an explosion that destroyed Vilgax once and for all, therefore avenging the deaths of both Gwen and Max. Ben transforms into Alien X and remains trapped in his powerful newfound alien form. Devastated at the deaths of his cousin and grandfather, he decided to used his new alien to travel throughout the Omniverse to hunt down his alternate counterparts to make them suffer the same way as him by taking away their Omnitrices, as well as occasionally destroying or severely injuring them.
Prior to the events of the episode, Maximilian and 15 year old Ben Prime have been chasing Alien X down.
Alien X obtained a Omnitrix from an alternate timeline where Gwen never joins Ben on a summer road trip with Max, before travelling to the Reboot timeline. The Reboot Ben transforms into Four Arms, Heatblast, Humungousaur, Diamondhead and Rath, but those aliens was defeated by Alien X, proving to be too powerful for any of his alien to fight, and ends up losing his Omnitrix to Alien X. However, Ben eventually reobtain his Omnitrix with the help of his family and his other alternate counterparts, who also help him to remove the stolen Omnitrices from Alien X's body. Ben and his counterparts team up and successfully defeated Alien X with all their efforts. Omniverse Ben used Bloxx to handcuffed Alien X after being weaken from having the Omnitrices removed. Alien X finally reverts back into Ben and was taken to the Null Void by Maximilin Tennyson (alternate counterpart of Max Tennyson) as punishment for crimes against other Bens.