Well, the title says it all. I will be describing how to use the skill here.
Spectral Dagger is a very nice skill that has a hidden use. It can allow you to pass through objects may it be creeps & heroes or may it be buildings, cliffs & forests. Cast it in a direction, then spam right-click a short distance (about 100 range) over the path and Mercurial will follow. You might want to carry around a Scroll of TP before you get BoT so you can disappear into the woods then teleport to the fountain. This has many uses, so be creative.
Now, how to use this special skill? Consider the situation:
You have to move from point A to B. There are two ways of doing this. You can right at B and follow the longer path or you could use spectral dagger and reach b much more quickly through the shorter path. I’ll explain you how:
Release the dagger in desired direction (towards B), and spam right-click short distances in the direction. Do not simply right click B after using dagger because then spectre won’t use the shorter path. You can also do SHIFT + right-click spam to make sure spectre follows the path.
This is extra useful when you find yourself in sticky situations such as ganks, and a good way to escape if you notice the gank in time is to run into the forest where the opponents cannot reach you and teleport out.
I hope you remember this post next time you play spectre.