In Season One, Episode Thirteen of Arrow, Slade Wilson was introduced. My life was over because superhero nerds know that Slade Wilson is the iconic DC villain known as Deathstroke. People come into close combat conflict with Deathstroke tend not to live to tell the tale. He’s an expert martial artists and he’s very strong due to being injected with a drug called Mirakuru which means miracle. His preferred choice of weaponry is a ninja-like samurai sword. Mirakuru is a Japanese super-soldier serum, much like what Steve Rogers/Captain America was injected with but worse.
It causes an exponential increase physical strength, speed, endurance, durability, agility, stamina, reflexes, plus an accelerated healing factor once injected. This drug can make you or break you. In a nutshell you will become all of the above or it will kill you. It varies from person to person. One other to have survived it is Roy Harper/Red Arrow.
Throughout Arrow, there were constant flashback scenes of Oliver Queen, Slade Wilson and Sara Lance on the island.These ran into season two. Obviously Oliver escaped and became the Green Arrow but at a cost. Simply, a girl named Shadow died because of Oliver. He had a choice to save her or save Sarah and he chose Sarah. She died and Slade blamed him for her death. He escaped leaving Sarah on the island with Slade. Slade vowed that he’d make Oliver pay for her death and take everything that he holds most dear away.
Eventually, he arrives at Starling City and starts reaping havoc and thriving on the chaos is has created. He kills Oliver’s mother, Moira Queen. One step closer to destroying Oliver’s world. He nearly kills Felicity, Oliver’s computer assistant. She is Arrow’s reply to Fitz-Simmons in Agents Of S.H.I.E.L.D or Harrison Wells, Caitlin Snow & Cisco Ramon in CW’s The Flash.
In order to defeat Deathstroke, Oliver Queen needed everyone’s help. He led an assault with SCPD (Starling City Police Department) behind him with added assistance of Roy Harper/Red Arrow, The League Of Assassins (led by Nyssa Al-Ghul), John Diggle and Sarah Lance/Black Canary. The final battle comes down to a single combat fight between Oliver and Slade on a rooftop. Oliver wins and Deathstroke is defeated. He is now locked up on the island where he and Oliver escaped.