Krell was a Klingon fleet admiral who entered into a covert alliance with Harris, a Human Section 31 operative, in 2154, to eliminate the Klingon augment virus which was sweeping through the Klingon Empire.
Convincing Harris that, if Harris facilitated the capture of Doctor Phlox in order to force him to assist in eliminating the plague, both the Empire and Starfleet would benefit, Krell had Phlox delivered to Qu'Vat colony to assist Doctor Antaak in developing a cure. Harris later contacted Krell to warn him that Enterprise NX-01 was en route to the colony, and Krell revealed that he intended to destroy the colony and Enterprise, and that he never intended to honor their original agreement.
Krell later arrived at the colony with three Klingon battle cruisers, and attempted to destroy the colony, despite Phlox's claim to have developed a cure. Krell's fleet engaged in battle with Enterprise and the Columbia NX-02, eventually gaining the upper hand. Krell contacted Captain Archer and proclaimed that both Starfleet vessels were now the property of the Empire. As this was happening, however, Phlox beamed a canister of the virus aboard Krell's ship. Contacting Krell, Phlox offered to share the cure with him if he halted his plans to destroy the colony. Having no choice, Krell relented, and spared the colony and the Starfleet ships. (ENT: "Affliction", "Divergence")