Thanagarians are a race of extraterrestrial beings from the planet Thanagar.
When Vandal Savage did not unite Earth, the Thanagarians destroyed the world. 
In Ancient Egypt, during the time of Khufu and Chay-Ara's first death, Earth and Thanagar became aligned, and a meteor fell to Earth which gave Hath-Set immortality and Khufu and Chay-Ara the ability to reincarnate.  Another meteor fell in Harmony Falls in 1958, and Savage used it to mutate Tommy Fuller and Jefferson Jackson into Manhawks. The final meteor was collected by Savage in 2021, but flown into the sun by Rip Hunter.
After the destruction of Oculus the Time Masters were unable to calculate the future of the current timeline, citing the Thanagarian invasion as a possibility that may or may not happen. Due to this change of plans, the Time Masters withdrew their support of Savage's cause. 
DC's Legends of Tomorrow
Behind the scenes
- In DC Comics the Thanagarians were the population of planet Thanagar. They were a race of bird-winged people, with their traditional attire resembling Hawkman suits, it's however unknown if it's the case in the show. The Thanagarians were aggressive and dreamed of turning the entire universe into a fascist police state mirroring their own.