Starting Items
Later goes into Basilius/Aquila.
You can't buy this from the side shop, so get it now.
Your first cast of Venomous Gale will take about half of your mana.
Early Game
Turn off the non-hero aura if you don't want to push the lane.
Get these as fast as you can, your starting speed is awful. If you can't seem to get the gold, make someone else get wards until you can.
Core Items
You won't need these for the whole game. When your mana problems become less of a big deal, disassemble these for Boots of Travel.
All of your damage is magic, and this amplifies magic damage in a large area.
Helps with your poor mobility.
Situational Items
There is likely a better mek carrier on your team, but if there isn't then get one.
Even more damage for your ult, and on a shorter cooldown.
Remove silences, dodge projectiles, stop TPs. Plus you solve any mana problems and get some much needed move speed.
Your only disables are slows, so if you need some more versatility, this is a good choice. It also takes care of all mana problems.
A longer range slow so you can catch up and apply your closer range slows. Nobody without a haste or blink is getting away.