Dagger missions are available after you complete the prologue and start free roam. There's one nearby at the beginning, called The Mithril Blade. In this mission, you stealth kill ten of about fifteen orc slavers in three minutes. It's very easy. Watch SoshiPanda Gaming's video below for more advice.
https://www.youtube.com/watch?t=82&v=2uuibniNQ2M