Drain is the main skill you should upgrade. Period. It allows you to heal without having to buy health potions, and it can also damage things in the process. While other heroes are wasting gold on health potions, you can just sit there and drain their minions for the same effect. The reason I have dark wind as the level 2 skill is because it is really annoying especially against squishies. It silences and it can also potentially rape their health if you can throw it at the right time. I then upgrade drain again for higher damage, and then move on to get level 1 of terrify. Terrify is not important to have high leveled until later on, because at level 1 it allows for 1 second of CC, which is more than enough to secure a kill or escape an enemy.

When you reach level 6, if you are not mid, immediately hide in the bushes. Most of the time, the enemy will not realize you've reached level 6, and so you can run in the bush, wait until they're close enough, ult on their faces while your teammate comes in to dish out extra damage, and score at least 1 kill using terrify and drain right after you ult on someone. The trick with ulting when it's still a laning game is to aim for one person, rather than try to kill both people in your lane. If you can, great! If not, don't risk popping your ult for nothing.