By 2154, Fleet Admiral Krell held a high position within the Klingon Defence Force and was dispatched by the Empire to deal with colonies that were affected by the Qu'Vat virus. His task brought him before the Qu'Vat colony where he attempted to destroy the colony once it became infected by the augment metagenic virus. At he time, he was also secretly involved with the Human intelligence agency known as Section 31 and its operative Agent Harris. Despite Doctor Phlox treating the virus and making it harmless, the Admiral was determined to destroy the colony but stopped when Phlox beamed a canister of the virus onto Krell's bridge thus infecting him and his crew. Krell was only cured when he powered down his fleet's weapons, after which he convinced the Klingon High Council to end its campaign of sterilization. He then began to distribute the cure to the rest of the Empire. (ENT episode: "Divergence")
As a result of his infection which turned him into a QuchHa', Krell's influence within the Empire had diminished and had even vowed to kill the Denobulan Phlox for his involvement in the matter. In 2155, Krell recieved a communique from Earth Captain Jonathan Archer who demanded an explanation of apparent Klingon attacks on Coalition of Planets space. Krell refused to dignify the Human with a response but later contacted Archer once again when the Captain threatened to reveal to the Klingon Empire his secret connection with Earth intelligence agency Section 31. The Admiral informed the Human that they were not involved in any attacks against the Coalition and when three Klingon battlecruiers were seemingly attacking Draylax, Krell dispatched vessels to destroy the seemingly rogue ships. He later discovered that the ships had been taken over by the Romulan Star Empire through a form of remote control technology though did not reveal this find to Captain Archer or the Coalition as it was considered a source of embarassment.
Krell was later summoned to Qo'noS by the Klingon High Council and Chancellor M'Rek to fight in one-to-one combat against Captain Archer who came to the Klingon homeworld to learn the truth about the attack on Draylax. The Chancellor informed the Human that he would only get the information if he defeated Krell. Captain Archer was successful in defeating the Admiral after severing his arm off but spared his life. The Admiral was then treated and had his arm successfully reattached by his husband, Doctor Kon'Jef though he would never have full use of it again. Defeated by the Human in combat, the Chancellor informed Krell to reveal the truth about the Draylax attack and the Romulan involvement in the matter. (ENT novel: Kobayashi Maru)
In 2156, Archer returned to the Council in order to ask for the Empire's assistance in the Earth-Romulan War. Krell was present at the meeting of the council and constantly traded barbs with Archer. (ENT novels: Beneath the Raptor's Wing, To Brave the Storm)
In the following years, Krell served as M'Rek's closest advisor. After the Chancellor died in 2165, Krell and the other Council members requested Doctor Phlox's assistance with the post mortem examination, seeing that they needed an impartial outsider. After the Denobulan doctor discovered that M'Rek was murdered and discovered the identity of the culprit, Krell took it upon himself to to deal with the murderer. After executing Krit, son of Antaak, Krell performed the Mauk-to'Vor with his husband's assistance, for failing to protect his Chancellor. (ENT - Rise of the Federation novel: Live by the Code)