Pros and cons
[+] Easy early game start.
[+] Early dueling.
[+] Bigger threat to any hero.
[+] Allows for solo roaming and easy dueling.
[-] No blade mail.
[-] Can Go very poorly if you're a poor aim with Legion's first.
[-] No Room for aegis most of the game.
Why Mid Lane?
-Because of the massive cost for Legion's 1st you would buy a bottle in order to keep legion's mana stable and full.
- Early XP.
- Enough Gold to provide you with chasing material.
How do you play a legion?
When playing legion I personally harass early game using my first and use bottle in order to extend that harassment allowing me to hook creeps and well as heroes in my storm of 1sts. Once you get duel you should start roaming around preparing to fight. When you see an opening ping your team mates and use your first for the extra movement (Make sure to aim for a lot of creeps along with the hero for bonus damage). Continue doing this method until you get shadow blade in which case you want to start aiming for supports or certain carries which aren't total tanks (Make sure to get your first hit off BEFORE dueling for the shadow blade's extra damage). Once you get your desolator you now have the game within your hands. With duel I suggest targeting hardcore supports or any unit that would destroy your team in a fight. When it comes to picking the final item NEVER get blademail seeing how at this point you don't want to use the extra seconds to give them time to activate skills and because blademail won't do much as this build will most likely cut through their health like a knife through Roshan's Cheese. In personal Opinion I recommend HoT seeing how it'll allow you to stay in lane longer and give you an easier time taking racks.