Fomorian

Fomorians are a class of Grineer battleship, considered the largest and most powerful vessels in the Origin System bristling with immense firepower and formidable armor, and a single one is capable of defeating an entire fleet of ships. Their power has made them the repeated targets of the Tenno that seek to stop Grineer influence from spreading.

Operation Sling Stone
Fomorian-class ships were first revealed in a transmission by Alad V to the Corpus Board of Directors, reporting of Grineer threats against the Corpus by deploying a new class of "supership" against them, much to Alad V's disbelief. The Lotus however is more wary of the threat, confirming the existence of the Fomorian threat and their imminent invasion of Corpus space unless they can be stopped.

To this end, the Lotus send the Tenno to destroy the Fomorian ships by using special Corruptor substances gathered from fallen Infested injected into the ship's reactor core, which will destabilize the cores and ultimately lead to the Fomorian's demise. The Tenno succeed in destroying all existing Fomorian ships, ending the immediate threat against the Corpus.

===Operation Tethra's Doom Several months after the last of the Fomorians were supposedly stopped, intelligence gathered by the Lotus reveal that Councillor Vay Hek is rebuilding the Fomorian fleet under Project Tethra, which aims to produce thousands of Fomorian-class ships to dominate the Origin System.

In order to hinder the progress of Project Tethra, the Tenno are sent to find the factories where the Fomorian Power Cores are being created. The locations are kept a closely guarded secret by the Grineer however, which leads the Tenno on Interception missions on Earth to tap into Grineer transmissions and determine the source of the Power Cores. Once the factories have been located on the shipyards of Ceres, the Tenno are to hijack the cores and then send them via Liset out into deep space, where they can be destroyed in safety.

While initially successful at destroying large numbers of cores, a gloating Vay Hek reveals that the Tenno have only been dispatching auxiliary cores instead of primary cores as believed. The Tenno intensify their efforts in response, identifying the locations of more important Shield Cores and destroying them en masse. The damage dealt by the Tenno hinder Project Tethra greatly, but fail to completely stop the production of the Fomorians.

Balor
With the precious time given by Tenno operations against Project Tethra, the Lotus seeks new countermeasures to defeat the Fomorian threat. Events come to a head however when the Tenno, in their search to find information on Vay Hek's new Balor-class Fomorians, are caught in a battle between the Corpus Ship they're on and a newly deployed Balor Fomorian. With the arrival of his new ships, Councillor Vay Hek declares war on the Corpus and Frohd Bek, signaling the start of heightened conflict between the two sides. Caught in the crossfire, the Tenno escape from the scene using their experimental weapon against the Balor Fomorians: the Archwing.