Invisibility allows a Huntress to build traps, repair towers, or attack without much fear of being targeted. However, Invisibility does not prevent the Huntress from incurring damage when she intercepts enemy projectiles or stands within the range of an enemy's AoE attack. In PvP, Invisibility prevents creatures from seeing the Huntress but does not prevent players from doing so.
Leveling and Costs
Putting points into this skill will decrease the amount of mana it takes to upkeep the skill. If not leveled, you'll need 15 activation and 4 mana per second which will slowly increase the longer it is used.
One of the downsides of Invisibility is that it will consume a small amount of mana every second it is active. This can be particularly bad on maps where you need to conserve mana as it will keep picking up every mana crystal on the floor around you and possibly waste them (if you are overfilling your mana bar). As such it is best to use Invisibility only when it is needed or if the map has an abundance of mana.