Watch your cooldowns, as they are long and unforgiving -- especially if you find yourself in an unpleasant situation. Unstable Poison and Festering Wounds are huge power spikes for early game, and gives you unbelievable wave clear + poke damage (also easy quest progress). With Festering Wounds it is very easy to get picked off if you aren't paying attention, since Haunting Wave is your only disengage until Level 16/20. Don't forget to spread your curse with a basic attack when you throw your Shadow Dagger at an enemy.

If you want to instantly clear a wave, use Haunting Wave, throw Shadow Dagger at the center of the wave, and kill the minion you dagger'd to kill the rest in a cascade.

Level 4
Most talents at this tier are very viable. While I prefer the wave clear that Unstable Poison provides, the additional lane pressure that can be gained from Possession can be even better in some cases. Mercenary Queen is very helpful for taking camps and is very good on Towers of Doom.

Level 13
Taking Windrunner instead of Cold Embrace allows you to play much more aggressively with your Haunting Wave, as you have extra escape after using your ability to engage or dive.

Level 16
If you find yourself being targeted a lot, Will of the Forsaken is a good talent to give you some extra escape.

Level 20
Bolt of the Storm can be another decent disengage option, and can also be used for engage if you can get away with it.