Your main source of damage and maybe income. Use to harass people out of lanes. Try to get as many creeps and heroes as possible in the AoE. Use in team fights on 4-5 heroes to get insane damage bursts. 180+65*5= 505 damage which can be better than a lion's ulti with aganim...
Get one point in this asap. Try to get debuffs off of teammates and yourself. (like CM frost)
We level this before press the attack because it's useful in duels.
Get your duels in. Don't worry about your kills going to other people, just get that damage bonus and get out with shadow blade if things get hairy.
Early Game
Use for setting traps on the lane. Get out of enemy sight, use the item and lie in wait. Tell your lane to pull the lane back if they can.
Core Items
First item you should go for. Use for escapes and to get people with your ulti.
This one before hyperstone if you can't farm fast enough. If you can get the 2k then go for hyper.