Mass Effect: Dawn of the Reapers is a Mass Effect based mod that tries to remain as close to series canon as possible.
Citadel - Made up of various different species united under one government, the Citadel forces have both strong military might and an influential diplomatic system, letting them strike with either the sword or the olive branch. With the advent of war with the Reapers, as well as the sudden surge in Cerberus activity, the Council has decided to allow the Quarians and Krogans readmittance to the Citadel.
Cerberus - What began life as a rogue Alliance black-ops organisation has grown into a xenophobic terrorist organisation, headed by the Illusive Man. Cerberus shares many common features with the Alliance, and by extension, the Citadel forces. Ruthless and determined, Cerberus focuses heavily on advancing their technology, as well as infiltration and subterfuge.
Reapers - Little is known about the Reapers, except for their incredibly advanced technology and their desire to extinguish all advanced life-forms. More powerful than any foe ever faced before, the Reapers are the pinnacle of all life.