Highly evolved Apex Predators on their homeworld of Parnack, the Yahg are a brutal sapient species deemed by the Citadel Council as incompatible with wider galactic society. Towering creatures and rather imposing, they are said to put the Krogans' savagery to shame. Beyond their massive size, strength and aggressive nature, however, they are also a race with seemingly limitless learning potential.
Though highly intelligent, the Yahg evolved with with their more animalistic behaviours intact, they are an extremely violent species and exhibit a strong pack mentality. This mentality is so strong that when a leader is established - whether through violence or social manipulation - all others will fall in line behind it, including any former rivals or enemies.
The Council discovered the Yahg at a technological state equivalent to that of the Human race's 20th Century status. They attempted to make first contact but the Yahg were insulted that they would consider themselves equals and not subordinates. This quickly led the Yahg to massacre the Council's entire contact party. The Council then quarantined the species from the rest of the Galaxy, determining that due to their size, brutal nature and obsession with control that they would not be able to integrate with the galactic community without incident.
The Shadow Broker
Only one Yahg is known to have made it off Parnack, a Yahg whom was revealed to be The Shadow Broker in Mass Effect 2's Lair of the Shadow Broker DLC. Liara surmises that this Yahg had killed its predecessor and then taken his place as Shadow Broker for the following 60 years.
It is revealed through mail archived on The Shadow Broker's ship that his predecessor had sought to use the Yahg as his own personal shock troopers or bodyguards. Smuggled willingly away from Parnak by Operative Kechlu, within merely a few months of arriving on The Shadow Broker's ship this Yahg exhibited the incredible learning potential of his race, learning seventeen languages and making political predictions that bettered some of The Shadow Broker's best agents. By the end of those same few months the Yahg murdered Operative Kechlu and assumed his duties - a course of action that had been anticipated by The Shadow Broker.
Soon, however, The Shadow Broker determined that the Yahg was too great a threat and attempted to order it killed. While in the process of doing so, however, the Yahg managed to kill The Shadow Broker and replace him without his subordinates knowledge thanks to the inherent secrecy surrounding The Shadow Broker.