Commander Maro

Commander Maro is a character in The Elder Scrolls V: Skyrim and the main antagonist of the Dark Brotherhood questline. He is the leader of the Emperor's special security force, the Penitus Oculatus, and is trying to destroy the Dark Brotherhood and protect the Emperor from any threats, which include them. Alternatively, if the Dragonborn chooses to become enemies with the Dark Brotherhood, Maro becomes an ally and assist them in massacring the Brotherhood.

Although initially calmer and seemingly more rational, Maro is affected by the murder of his son Gaius, and develops a hunger for vengeance against the Brotherhood. At the last mission of the questline, he is killed at the Solitude docks, and the Emperor is assassinated shortly after.

Background
Not much is known about Maro's past. He hails from the Imperial Province of Cyrodiil and at some point before the events of Skyrim, joined the Penitus Oculatus special security force and rose through the ranks until becoming Commander. He is very dedicated to his job and is willing to protect Emperor Titus Mede II's life at any cost.

Maro has a son named Gaius, who is also a member of the Penitus Oculatus, and is quite close and protective of him. However, he accepts that Gaius is a man and allows him on important missions alone, though still giving him very specific instructions before doing so.

Dark Brotherhood storyline
During the game, the Dark Brotherhood receives a contract from Amaund Motierre worth of thirty thousand Septims: assassinate Emperor Titus Mede II. Astrid, the leader of the Brotherhood, plans on attracting the Emperor to Skyrim through the murder of his cousin, Vittoria Vici. After this is done, Astrid tells the Listener (the Dragonborn) about Maro, and that they need him to be distracted for the assassination to work.

To achieve this, the Listener murders Maro's son and leaves an incriminating letter on his corpse, framing him for a plot against the Emperor's life. Not only does this devastates Maro, but it also gives the Emperor a false sense of security. However, Maro instead gains more motivation to hunt his son's killer. He somehow learns of the Dark Brotherhood's involvement, threatening to massacre them if they do not let him kill his son's murderer and foil their plot. Intimidated, Astrid agrees, and Maro promises to retract all aggression against them.

When the Listener assassinates the "Emperor", Maro confronts him shortly after and reveals that his target was just a decoy, and that someone in the Brotherhood betrayed them. He then reveals he won't fulfill his part of the deal, and orders his men to kill the Listener while he goes to the Dark Brotherhood Sanctuary to burn it to the ground. When the Listener arrives at the Sanctuary, it is too late, most of his fellow assassins are already dead and the Sanctuary is in flames.

However, the Listener and two other assassins survive the massacre and decide to fulfill the hit on the Emperor to rebuild the Brotherhood with the reward. At this moment, Maro is overseeing the Emperor's departure from Skyrim on the Solitude docks, and while the main objective is to kill the Emperor, the Listener may optionally assassinate Maro on the way, avenging their fallen brothers and sisters.

Destroy the Dark Brotherhood
Alternatively, if the Dragonborn attacks Astrid as soon as she approaches them, they may speak to Maro about the Brotherhood. Maro is cheerful with Astrid's demise, and tells the Dragonborn he'll send a squad of soldiers to the Sanctuary to help him destroy the Brotherhood for good. He gives three thousand Septims as a reward, and remains friendly for the remainder of the game.

Trivia

 * Maro is the only main antagonist of a questline in Skyrim who may be spared. Alduin, Mercer Frey, Ulfric Stormcloak/General Tullius, the Silver Hand, Ancano, Lord Harkon and Miraak must all be killed in order to complete their respective questline.
 * Along with Ulfric and Tullius, Maro can be seen as one of the only primary antagonists who is "redeemable", with some noble goals but flaws. All the other antagonists either want power, destruction, self-gain, domination or racial genocide.