King Nergill is a minor villain from the original G1 Transformers cartoon who only appeared in the 2nd season's episode "Atlantis Arise". He is the king of Sub-Atlantica who wants to take over the Earth.
When the Decepticons arrived on Sub-Atlantica, King Nergill greeted them but Starscream, distrusting him & his kind, opened fire before being stopped by Megatron. King Nergill then showed his kingdom's energy storehouse & advanced technology, which impressed the Decepticon leader. Megatron & King Nergill made a deal in which the Sub-Atlanticans will provide the Decepticons energy while they in turn will help them in turn conquer the surface & wage war on the land dwellers. Only Starscream distrust him & his people.
When the Autobots came to investigate Sub-Atlantica, the Sub-Atlanticans ambushed them with Decepticon help & manages to capture Wheeljack, whom the king ordered to be imprisoned in his dungeon. However, King Nergill then secretly orders to experiment him in the laboratory instead. Starscream finds out about this, thanks to Soundwave encypting his telepathy. The Decepticons then raise Sub-Atlantis, then the two forces conquered Washington D.C., as a sign of new allegiance. Afterwards, the Sub-Alanticans raised a force field to imprison Optimus Prime's Autobot team & preventing backup from arriving.
Back at Sub-Atlantica, King Nergil had just finished experimenting Wheeljack by using his experiment magnetic dysfunction ray on him, leaving him immobile. He is secretly planning to use this on the Decepticons after successfully conquering the Earth's surface. Starscream finds out about this & tried to stop this but was shot by the latter. He then leaves Starscream to arrive at Washington D.C.
King Nergill then strolls into Washington, while the Autobots and a freed Wheeljack, tailing on him. He then uses the disruptor on Optimus Prime's team, disabling them. While securing the Capitol City, one of the Sub-Atlantican soldiers ask him when to move against the Decepticons, the king then states to wait a little longer. Wheeljack then summons the Dinobots, who were strong enough to break the Sub-Atlantican force field, arrived & attacked the Decepticons. As Megatron & Grimlock fight, King Nergill tries to shoot both of them, only to be stopped by an escaped Starscream who claims he is an enemy. Both forces retreated to Sub-Atlantica as the Autobots pursues them.
Megatron then demands Starscream's treason against the Sub-Atlantican king but the latter warned him of the incoming Autobots. The Dinobots arrived & started destroying the city. King Nergill then decided to destroy Sub-Atlantis, hoping he can destroy both Autobots & Decepticons before they could. Spike, Wheeljack & Bumblebee pursuits him, while Ravage & Rumble followed. The two Decepticon Mini-Cassettes realized his true intentions & Rumble tries to bury the king in rubble, only to fail. Wheeljack tried to stop him but King Nergill threw a device into the energy chambers, starting a chain reaction that begins to destroy the city. After both Autobots & Decepticons left the city as it sinks into the ocean, Optimus stated that they won't be seeing Nergill or his people ever again.