Known for their militaristic and disciplined culture, the Tieflings were the third race to join the Council. They gained their Council seat after defeating the hostile Orcs for the Council during the Orc Rebellions. They deployed a Gnome-created biological weapon called the genophage, which virtually sterilised the Orcs and sent them into a decline. They then filled the peacekeeping niche left by the once-cooperative Orcs, and eventually gained a Council seat in recognition of their efforts.
Originally from the planet Palaven, they are best known for their military role, particularly their contributions of soldiers and ships to the Citadel Fleet. They are respected for their public service ethic - it was the turians who first proposed creating C-Sec - but are sometimes seen as imperialist or rigid by other races.
There is some animosity between Tieflings and Humans, largely due to their role in the First Contact War. This bitterness is slowly beginning to heal—as shown by the cooperation of the two races on the construction of the ALS Normandy SR-1—but many Tieflings still resent Humans, and vice versa.