Krogan

The Krogan are the hardiest and theoretically longest-lived species known to the Milky Way galaxy, and by circumstance the most volatile. Evolved on the violent world of Tuchanka, the krogan enjoyed a harsh yet straightforward advancement until their society had reached the nuclear age. At this point, violence between clans no longer simply impacted themselves, but their world as a whole. Civilization was lost, bombarded into a radioactive, survivalist wasteland.

Nearly a millenia later, the krogan were discovered by the Salarians. The Citadel Council, thickly embroiled in their own Rachni Wars, quickly enlisted the help of the hardy combatants and used them to turn the tide in the conflict. After pushing the insects to extinction, the krogan were hailed as heroes and granted a new homeworld. As the krogan population exploded however, their government grew ravenous in its expansionist policies, culminating with the attempted annexation of the asari colony of Lusia. The Citadel Council made a preemptive strike on krogan infrastructure on several key planets, igniting the Krogan Rebellions. In the end, the krogan lost control of many of their own worlds and the Genophage was unleashed upon the krogan population, massively increasing infant mortality rate across the species.