The Empera Army is a vast army of monsters assembled by Alien Empera to assist him in his goal of galactic conquest and major antagonists in the Ultraman franchise.
Great Ultra War
The Empera Army fought the Ultras in the Great Ultra War 30,000 years prior to the Showa Ultra's arrival on Earth. Empera's forces eventually took control of the Land of Light, but this did not stop the Ultras from fighting back.
Alien Empera and the Father of Ultra eventually confronted and fought each other in an engaged in a duel to decide the fate of the Land of Light. The fight ended in a draw, with both fighters gaining mirroring scars on their hips. The Emperor, weakened, then retreated to begin rebuilding his strength
The Great Ultra War is mentioned in episode 25. In episode 33 and 34, a group of Alien Temperors arrives on Earth to hunt down the Ultra Brothers. Early magazine publications reveal that these Tempers are servants of Alien Empera.
After hundreds of years of laying low, the Empera Army began a campaign to invade the Earth.
After the destruction of the Emperor's top generals, the Four Heavenly Kings, Empera himself arrived on to Earth, encasing the sun with darkness and personally fighting the Ultra Warriors Mebius and Hikari. However, the darkness around the sun was destroyed by the Ultra Brothers and Empera was defeated by Ultraman Mebius.
Ultraman Mebius Gaiden: Armored Darkness
Though Alien Empera was dead, the Emperor had a contingency plan in the Armored Darkness, a sentient suit of armor that continued the Empera Army's campaign until it was halted by Crew GUYS and Mebius and Hikari.
Ultraman Mebius Gaiden: Ghost Reverse
The Four Heavenly Kings were later revived as ghosts and began plotting to revive Alien Empera using his remains in Mecham.
They lured Mebius, Ace, and Taro into the Monster Graveyard after capturing Hikari. When Mebius finally arrived, Hikari told him he had joined the Four Kings. The Four Heavenly Kings then ordered Mebius to retrieve the Giga Battle Nizer so they could revive the Empera or Ace and Taro would be killed.
When Mebius returned, Hikari stole it from Mefilas and revealed he had only joined the Kings to rescue the other Ultra Brothers. A fight then broke out between the Ultra Brothers and Four Heavenly Kings where the Four Kings' plan were defeated. Their plan, meanwhile, was foiled when Mechazam sacrificed himself to prevent the Emperor's resurrection.