Slade is an ASIS Agent who was sent with his partner to rescue Yao Fei on an island. Their plane was shot down, and that is where Slade met Oliver Queen when he washed ashore after his boat accident. 

Slade, code name Deathstroke, begins to train Oliver on the island so that they might be able to escape. Oliver is captured by Slade's former partner and Slade kills his partner to save Oliver afterward they form a friendship. 

Slade turns up in Starling City to get revenge upon Oliver for the death of Shadow. A.R.G.U.S. has been calling him Deathstroke because of his worldwide killing spree.

Slade Wilson/Deathstroke Quotes

Sebastian Blood: You know who he is under that hood, don't you?
Slade: I know exactly who he is. He's my friend.

I will arrange for you receive another sample of my blood, and then you can begin to mass produce the serum again.When I ask you not to confront the vigilante, you will listen. Remember, your mask can be worn by another.

Slade

Arrow Quotes

Diggle: I see you brought your plus one.
Ray: Nice to meet you. Ray Palmer.
Diggle: John Diggle. [whispering into Ray's ear] You hurt her, they'll never find your body. [louder] Nice to meet you!

Felicity: What's going on? Ray?
Ray: Oliver Queen is the Arrow.
Felicity: What?
Ray: I have a 140 IQ and three PhDs, but it's pretty hard to insult my intelligence, but I'm pretty sure you just did. It all makes sense now. Your, ah, sudden disappearances, your apprehension of helping me with my mission. You've been working with the Arrow this whole time.