Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This build is all about fearing and draining/winding, falling back a little, rinse and repeat. When you see opponent is 50% hp, its time to fear and crow his butt. It's important to have good AP but to stack some HP items too, because Fid is sooo squishy.
Flash/Ghost are good evasion spell/crow opener. Make sure you drain immediately after you pop your ultimate because you're gonna be the target of the moment.
Fear is your lifesaver. You must spam it as soon as the cooldown is over. Dark Wing is good for farming at the begining and to silence casters. It's a very annoying spell but fear/drain are way better, that's why I invest in them at start. Make sure you fear everyone that is getting even remotely close to you. No one gotta be in your "bubble".
The items linked here are pretty much the items you're looking for. It's a mix of HP/Mana and AP.