Sir Marcus Antessio was a noble of House Mecetti. He was knighted in his early twenties, ostensibly due to his service in the House Defense Force. Considered particularly handsome by Human standards, he was a popular figure in the sector and a media darling, often pursued by HoloNet reporters to whom he showed a sort of indulgence, which increased his popularity with the commoners.
In secret, however, Antessio had also been trained as a member of the Mecrosa Order, and it was his service to that secret society which earned him his knighthood. He had some degree of Force sensitivity, which enabled him to employ some Force powers such as Force push and also sped up his combat reflexes. In addition to his military training and skill with common arms, he was also trained in the use of poisons and unconventional weapons, and used an Echani double-brand laced with cortosis weave as his weapon of choice.
Antessio was trained by Lady Menal Bathiriat, the head of the Mecrosa. Too young to contribute substantially to the Second Imperial Civil War, Antessio had some activity during the aftermath and the hunt for members of the One Sith. The Mecrosa opted not to ally themselves with any of the Sith claimants of the time after Darth Krayt's death, waiting to see whether Darth Nihl, Darth Talon, or another emerged as a stronger claimant. Antessio supported this ambivalence, enjoying the prospect of playing all sides against one another to benefit House Mecetti and see who came out the strongest; having studied records and teachings of the Sith, he felt this approach was in line with the values of the Mecrosa. In this he often argued with fellow Mecrosa knight Evangelia Maxico, who favored supporting a Sith claimant faction and helping the One Sith recover from the death of Krayt. Though adopted by a Mecetti baron, Evangelia had been raised to nobility from the common class of her birth, and Marcus loathed her privately as well as publicly disapproving of her strategies.
In 154 ABY, Mecetti spies discovered that Synix Fer'faloon intended to sell the holocrons of two Sith Lords on the Coruscant black market. At Lady Bathiriat's command, Antessio led a team of Mecrosa assassins, including Maxico, to Coruscant to obtain the holocrons for the Order's own use.
Antessio and Maxico argued about tactics on the way and on Coruscant itself, debating how best to find the holocrons. Where Maxico wished to pursue a path of information through the channels Fer'faloon had established, Antessio preferred to find others involved in the hunt and shadow them, killing them and taking the prize once the hapless enemies had found the holocrons. As the divide between them grew, Antessio finally set off with Thosso to find someone worth pursuing.
What he found instead was the Jedi Master Halambo and his apprentice Zee Pek. Surmising the Jedi were after the holocrons, Antessio decided to remove them from the equation, engaging Halambo in a duel while Thosso pinned Zee Pek down with blasterfire. Antessio took a few minor scratches, but managed to hold off Halambo until Zee Pek deflected a blaster shot his way. Antessio dodged the shot, but in his moment of precarious balance, Halambo activated the dual-phase function of his lightsaber and impaled Antessio through the throat. Paralyzed by the blade lancing through the back of his neck, Antessio collapsed and suffocated in a few seconds.