Augustus was born and raised on the Enclave Oil Rig. His father, Autumn Senior, was a high-ranking scientist in the Enclave at the time of the destruction of the Enclave Oil Rig by the Chosen One in 2242. He then received orders from the new President of the United States who succeeded Dick Richardson, a former presidential advisor named John Henry Eden, to take all surviving Enclave troops and relocate them to the Raven Rock military base on the East Coast. Autumn Senior obeyed that order. Augustus' father gathered his troops and their equipment and led his men eastwards through the wastelands of the post-nuclear United States. Upon the Enclave's arrival in the Capital Wasteland, he was the only one who knew that John Henry Eden was actually a ZAX supercomputer unit that had achieved self-awareness. Prior to his death, Autumn Senior passed that knowledge on to his son, who would become the leader of the Enclave's military and eventually attain the rank of Colonel.
Colonel Autumn has proven to be far less subservient than his scientist father, often openly disagreeing with the president's decisions. He did not trust Eden completely, fearing some of the supercomputer's methods were too extreme, and kept the ZAX Destruct Sequence for the President in his room as a last resort. At some point, he ordered experimental testing on ED-E be carried out while the robot was fully functional, to the shock of Dr. Whitley.
During the Enclave assault on Project Purity, Autumn confronted James and seemingly died after the scientist flooded the project's main chamber with deadly radiation to prevent the Enclave takeover. However, he managed to survive by injecting himself with a mysterious agent — likely a more effective equivalent of Rad-X or RadAway.
Colonel Autumn is extremely ruthless and shows no regret over actions, such as shooting unarmed captives or breaking promises of mercy, as long as it serves the Enclave's ends. He is a firm believer in the revival of the United States as a nation and a power in the post-nuclear world, and will do almost anything he has to in order to fulfill this vision.
The tension between President Eden and the Colonel finally snapped Autumn's loyalty, as he disobeyed the President's orders and usurped command over the human contingent of the Enclave's military forces at Raven Rock and relocated them to the Jefferson Memorial, to make their last stand.
As opposed to Eden's visions of eliminating all of the "impure" elements of the wastelands' population, Colonel Autumn's main objective seems to be increasing the Enclave's power over the remains of the nation once governed by elected representatives, as he intends to use the water purified by the Purifier as a means to make the people of the Capital Wasteland "flock to the Enclave" for leadership rather than killing them. Also, he is not completely heartless, as he is able to be reasoned with during the final encounter in Fallout 3 and will simply walk away. If Broken Steel is installed, you can hear Elder Lyons say "He/She just let him get away like that?..." during the 2 weeks later transition.
The Waters of Life: Colonel Autumn is first encountered by the Lone Wanderer during "The Waters of Life", locked in the Project Purity control room at the Jefferson Memorial with James and Janice Kaplinski. He declares the Enclave's intention to confiscate the project, and, in a show of his deadly seriousness, kills Janice in order to force James' cooperation. James, instead of shutting down the project, floods the area with radiation from the Purifier, in an attempt to kill Autumn and his men. James and the Enclave soldiers fall to the radiation and die, while Autumn seemingly injects himself with advanced RadAway or Rad-X.
Finding the Garden of Eden: Colonel Autumn is seen again at the end of "Finding the Garden of Eden". He kneels over the Lone Wanderer before he/she blacks out, having been knocked to the floor and stunned by an Enclave pulse-grenade. He orders his troops to secure the GECK the Lone Wanderer was carrying.
The American Dream: At Raven Rock, Autumn interrogates the Lone Wanderer, demanding the activation code for the Purifier. The player can give up the code 2-1-6 (in which case the Colonel will execute him/her using his pistol), tell him the wrong code, or simply insult him. In the case of the latter two, Autumn will be interrupted by Eden, who will release the player. Colonel Autumn later comes on the Raven Rock broadcast system, informing all Enclave troops that the player is a hostile target and to disregard the President's orders. He then escapes with a group of Vertibirds.
Take it Back!: Autumn is finally confronted by the player at the end of "Take it Back!", just outside the Purifier's control room once more. He is accompanied by his two personal bodyguards, a pair of high-level Enclave soldiers equipped with Tesla power armor and armed with Gatling lasers or miniguns. The player has the choice to either fight Autumn and his men, or convince him to leave peacefully through 2 consecutive speech checks of fairly high difficulty. One speech path involves convincing Autumn to abandon President Eden's insane plans and stop the war; if Raven Rock has been destroyed, another slightly easier speech path is available in which the player convinces Autumn he has nothing left to fight for.
It is possible that Colonel Autumn is using John Henry Eden as a puppet leader while he is really in charge. This is supported by the fact that only he talks with Eden face-to-face, which isolates Eden from the Enclave soldiers; while Lt. Williams was able to contact Eden directly, this contact was not in person. Another possible clue is when Autumn tells his troops not to listen to the president and to kill the Lone Wanderer, which they almost seem happy to do, as opposed to being reluctant to follow Eden's orders not to attack. This all seems to contradict what Autumn himself says when you try to convince him to leave the purifier however, as he says "I am sworn to protect the Presidency, the chain of command must be upheld." He also believes in Eden's authority, saying he was "next in line" for leadership, and you must convince him otherwise.
During the optional final fight with Autumn, if you kill one of his bodyguards first, he may pick up their minigun or Gatling laser, which he uses with skill, making him a much tougher foe.
After completing The Waters of Life, Colonel Autumn can be heard on the Enclave Radio informing the people of the Capital Wasteland of the Enclave's mission and warns them not to interfere with them.
When faced during the final fight his body may fall down the gap underneath the purifiers' control room, making his body unreachable and un-lootable. Killing him of course is optional.