He continues to serve the Imperial Legion until a certain point in the Dark Brotherhood questline, when he retires to Leyawiin. The Hero, as a Dark Brotherhood assassin, then tracks him down and assassinates him. He is succeeded by Giovanni Civello.
As said by Phillida's successor, Giovanni Civello, Phillida fought the Dark Brotherhood every day of his life and, according to the Dark Brotherhood member Ocheeva, Phillida tended to "make their lives uncomfortable" when their contracts took them to the Imperial City.
As Phillida had vowed to bring the leaders of the Brotherhood to justice, the Brotherhood targeted him for assassination twice, but failed on both occasions. Phillida reportedly uncovered thirteen separate attempts by citizens to contact the Dark Brotherhood. One of these included the unfortunate Claudius Arcadia, who was locked up in the Imperial Prison and whose house was seized and made into an Imperial Legion outpost. He defended the Imperial City for 40 years.
Adamus Philida is a very famous guard of the Imperial Legion. He is mostly known for his efforts against the Dark Brotherhood. Many praise him for his skills, by which he became Commander of the Imperial Legion and is known throughout Cyrodiil.
Adamus Phillida has pledged his life to destroying the Dark Brotherhood. Once the Hero accepts the Dark Brotherhood quest "Permanent Retirement," Phillida retires and takes up residence in the City Watch barracks just outside Leyawiin. He follows a strict schedule and is protected only by a single, inexperienced bodyguard. It is in this vulnerable state that he is being targeted by the Dark Brotherhood for the final time, both to remove an old rival, and to send a message to whoever follows him that the Brotherhood is not to be trifled with. The Hero of Kvatch is sent to Leyawiin to do the deed.