Jaroth was the leader of the Eclipse forces on Omega, and following repeated attacks from the Turian vigilante known as Archangel (AKA Garrus Vakarian), threw his lot in with the other two mercenary groups: the Blood Pack, led by Garm, and the Blue Suns, led by Tarak. Fancying himself the brains of the alliance, Jaroth organized their plan, which mainly involved sending in the freelancers as cannon fodder while the Eclipse and other two mercenary groups each tried their hand at taking Archangel out, also recognizing that the Turian was beginning to tire after spending so long on his own.
Jaroth held a personal grudge against Archangel, who had killed his brother while raiding an Eclipse transport. He also conspired with Tarak and Garm to overthrow Aria T'Loak, the leader of Omega, and take over the station for themselves after they had killed Archangel.
In the end though, Jaroth's schemes were all for nought. Commander Shepard and their allies came to Garrus' aid, and together completely wiped out all three of the mercenary groups, Jaroth and his Eclipse mercenaries included. Shepard can also possibly hack into Jaroth's YMIR mech and have it shoot at him and his own men instead of Shepard, Garrus and allies.
Mercenaries and other Criminals