Cuchulainn was a minor Goa'uld in the service of Morrigan. With the power vacuum that had been created because of Lugh's death that was quickly filled by Manannan mac Lir and Dagda promoted new General from their Underlord. Dagdas replacements were ambitious Cuchulainn who was very close to snapping of Morrigan that is carefully designed plans .
After Morrigan promised him much more power than he currently had in commanding a battle fleet, refused Cuchulainn and despised Morrigan as a "phantom queen" who lacked the capacity to make good on her offer. When this became Cuchulainn openly mock her with epithets, and private threatens to reveal his secret, Morrigan knew she had to deal with the threat once and for all. As such, she arranged for Cuchulainn's flagship is sabotaged during a critical battle. While this battle was extremely successful thanks to Morrigan own planning, Cuchulainn did not live to see the victory he had created in Dagda.