Slade Wilson

Slade Wilson, also known as Deathstroke, is a mercenary and assassin imprisoned on the prison island, Lian Yu and is one of the survivors including Yao Fei of a mass execution of the inmates by Edward Fyers and is later employed by Fyers to track down and kill Yao Fei.

Biography
When Oliver was stranded on Lian Yu, he was captured by Fyers' men who were looking for Yao Fei, their reasons unspecified. When Oliver refused to give up Fei's location, he was tortured by Deathstroke. Oliver is later rescued by Yao Fei, who engages Deathstroke in battle and temporarily subdues him; escaping with Oliver.

Abilities

 * Expert hand-to-hand combatant: Deathstroke is an expert in close-ranged hand to hand combat, as seen in Oliver Queen's flashback when he fought Yao Fei.
 * Expert marksman. Deathstroke was seen carrying an automatic rifle. While he never used it in the episode he appeared in, it can be assumed, that Deathstroke is an expert with it, along with a whole host of other firearms.
 * Master martial artist: Deathstroke is a master martial artist and is able to hold his own against Yao Fei who appears to be his equal.
 * Master swordsman: Like his comic book counterpart, this version of Deathstroke uses a katana-like sword as his primary weapon, and is extremely proficient with it
 * Peak human physical condition: Deathstroke is in top physical condition and  demonstrated incredible feats of strength, endurance, stamina, speed, and agility when he fought Yao Fei. Deathstroke also demonstated coordination, dexterity, and reflexes that appeared to border on superhuman.

Trivia

 * Deathstroke's mask was seen in "Pilot", mounted on a wooden stake when the Chinese fishermen who saw Oliver's distress signal came ashore to investigate.
 * Deathstroke has both of his eyes, specifically his right eye, which he does not have in the comics and most other media. This is expected to change due to the fact that mask mounted on the wooden stake seen in "Pilot" did not have a right eyehole.
 * Deathstroke gave Oliver his second scar by cutting the lower left side of his abdomen with a knife while torturing/interrogating him.
 * Unlike in the comics and other media, Deathstroke did not demonstrate his trademark regenerative healing factor. Since Arrow is heavily grounded in realism it is very unlikely that he has the same regenerative healing factor that he does in other media. Still it is possible the show's creators and writers will try to work Deathstroke's healing in.
 * The character has yet to actually be called by any name at all, so it is possible that his name is not actually Slade Wilson, though this seems unlikely considering how popular the character is.