The one of the items sets I and use the most is the abit more tanky one as the dmg from Bloodthirster, Blade of the ruined king and frozen mallet. And then I pick some survivability stuff just because I will be in the middle and I will take damage so adding some magic resist armor and the passive from spirit visage will help alot.

Pros
Very strong through out the whole game
Survives ganks easy
Amazing passive
Losing trades is not a problem with his W ability

Cons
Cant start fights late game
Forgetting to switching around BLOOD THIRST / BLOOD PRICE might cost you the fight
Weak against stuns

There's two diffrent ways I skill up spells with this champ it's either maxing BLOOD THIRST / BLOOD PRICE (if you're not getting pushed out of lane.) or BLADES OF TORMENT (if you have a really bad start / meet a counter.) first and the other one as second.

Do not ever use Dark Flight to farm if you're not a sure that you are alone at top as you might get a gank on you and without it they have a easier time killing you. Don't just spam Blades of Torment all the time without healing up with Blood Thirst as that will drop your health. Remember to take Golems when your lane is pushed if you're purple as junglers dont take them too often and they shouldnt do much dmg to you because of the lifesteal.

In teamfights don't jump in first if it's not a must try to get behinde the enemies and jump onto their AD and AP carries. Be aware of stuns etc that the enemy team has before you do anything as if you get stunned for to long you're going to die before you can even start life stealing.

Earlier on in the game in lanes you should try to hit the enemy player with your Dark Flight as soon as he sees your jungler and then use Blades of Torment as soon as he is out of your knockup.

This is how you want to start a fight if possible. You start with having Blood Price on then if possible use blades of torment to slow at start (this might not always be possible but you should go in after missing it so only use it if you're sure that you can hit anything with it.) and after that jump in with Dark Flight. Then you will use you're ultimate and start wrecking their ADC and APC. Remember to switch back to Blood Thirst when you start dropping i'd suggest just before you hit half hp as you get the bonus heal from it when you're under that. Don't forget your Blade of the ruined king on use it helps alot!