Masker

Maskers are supportive, stationary enemies appearing to resemble a priest. They cannot attack, but instead cover the vicinity in a dense fog, rendering other enemies invisible.

Behavior
Maskers don't move, can't be aggroed, and can't attack. The only thing they can do is make other nearby enemies invisible.

Behavior
Maskers emit a fog that is easy to spot as the player approaches. Enemies under the effect of this fog are not truly invisible, as their silhouette can be seen "camouflaged" in the form of a faint distortion seen at their location. Look out for these distortions so you have a better idea of where enemies are without getting ambushed. While Maskers are high-priority targets, approach it slowly and take care of the enemies in its way first.

If you have a ranged weapon, it helps to prematurely fire at invisible to get a better idea of where they are. Once you hit them, you can draw their aggro and get them close enough to reveal themselves.

Invisible enemies are also given away by traps and turrets as their targeting abilities are not affected. Giant Whistle cannot target invisible enemies, but it can target the Masker.