First Wizarding WarEdit
Rookwood worked as an Unspeakable in the Department of Mysteries, but became a Death Eater and spy for Lord Voldemort during the First Wizarding War. He had an information network inside and outside the Ministry of Magic with a number of well-connected wizards. Whether or not these wizards knew they were being used for information by Rookwood is unknown. What is known is that Ludovic Bagman gave Rookwood information, likely ignorant of the fact of Rookwood's real affiliations, since Rookwood dangled the prospect of a Ministry position before Bagman. Bagman's testimony revealed that Augustus Rookwood was friends with his father. He also referred to Rookwood rather affectionately as "Old Rookwood", despite the man being a Death Eater, during his own court case. From that information, it can be deduced that Rookwood was outgoing and perhaps affable during his years as a spy, breaking the mould of so many other Death Eaters, if only to serve as a cover.
In 1981 or 1982, Rookwood was captured and convicted on testimony from Igor Karkaroff as to his Death Eater activities and imprisoned in Azkaban. This was one of many names that Karkaroff surrendered in order to achieve his own freedom and the first one to be viable, as the ones before were already caught or killed.
Second Wizarding WarEdit
During the Second Wizarding War, Rookwood escaped Azkaban along with nine other Death Eaters in 1996, when the Dementors defected to the recently returned Lord Voldemort.
Rookwood was responsible for getting Voldemort on track with stealing the prophecy which concerned the Dark Lord and Harry Potter, who saw Rookwood in one of his dreams. Obviously, his experience in the Department of Mysteries made him a valuable asset in this function.
Battle of the Department of MysteriesEdit
After numerous attempts at stealing the prophecy failed, Voldemort lured Harry to the Ministry by sending him visions of his godfather being tortured there. Rookwood was one of twelve Death Eaters who participated in the ambush of Harry and his five friends and in the battle which ensued when the teenagers resisted. He was the only one not assigned a partner as the Death Eaters pursued the six Dumbledore's Army members. Rookwood's whereabouts for most of the battle were unknown.
What is known is that Rookwood was apparently uninjured when he entered the Veil Room, and was seen casting the Killing Curse at Kingsley Shacklebolt without his mask on. It is presumed that Rookwood, along with all the other Death Eaters aside from Bellatrix Lestrange, was captured and imprisoned in Azkaban after the battle.
However, he was again at large the following year, after another Azkaban break-out.
Battle of HogwartsEdit
During the Battle of Hogwarts in 1998, after Fred Weasley was killed in an explosion, Percy Weasley chased after Rookwood as he was attacked some of the other students who stayed to fight rather than evacuate the school. Later in the battle, he was stunned by Aberforth Dumbledore. It is unknown what happened to him after Voldemort was defeated, but if he survived, it is likely that he was captured and again sent to Azkaban.