Mabus was the Trabe elder who befriended Neelix and later used Janeway's peace conference as a chance to assassinate the Kazon majes.
Mabus led a group of Trabe, the people who once ruled the Kazon as an underclass until the Kazon rose up and drove them from their homes. Generations of conflict followed, leaving the Trabe as refugees hunted by their former slaves.
In Alliances, Mabus met Neelix when both were held prisoner by the Kazon, and his help earned Janeway's trust. He proposed an alliance and encouraged her to host a conference of Kazon majes aboard Voyager. Instead of seeking peace, Mabus used the meeting to attack and try to kill the Kazon leaders, and Janeway ordered the Trabe off her ship.