Slender is a known legend during the colonial times (I know it's a meme, but let's just pretend? :D). Both the Assassins and the Templars find out he really exist and decide that supernatural bogey man who steals children and does who knows what with them are bad for both their respective ideals.

So they enter into a brief and grudging alliance to get rid of the creep who is going after helpless children. Except...well, Connor's still a teenager. Something nobody really thinks about because he acts and looks so mature. And everything goes to shit when he becomes the next victim.