Krogan have very strange biological features. They have two hearts, four kidneys, two nervous systems, two penes, and four testicles. Apparently testicles are very popular on the black market. Krogan are willing to pay 40,000 Credits for a full set in order to increase their fertility, but it doesn't work anyway. Silly.
Krogan have a similar story to Brutes. They advanced their technology to a reasonable level then nuked themselves back to the stonge age, turning Tuchanka from a lush world into a shithole. They remained like this for hundreds of years until they were uplifted by the Salarians, similar to how Brutes were uplifted by the Covenant. They were used as soldiers in the Rachni Wars as only they could brave the conditions of Rachni worlds. Shortly after this they were granted membership to the League .
Krogan have big humps on their backs that act like storage tanks to store fat and water. As a result, Krogan can survive in even the most brutal environments. They also possess immense regeneration abilities, being able to grow back tissue in seconds and they can even regrow organs. Because of these features they are popular shock troopers and are utilised in many League Special Forces and mercenary squads throughout League Territory.