When you draw an Enemy, it appears at your location. 

Enemies at your location attack you, when you reveal a Skull (see Enemy Activation) and trigger Surveillance in the encounter phase (see Encounter Phase).

If an Enemy has Damage equal to or higher than its health, the Enemy is defeated and discarded. (This counts as completing.)

Effects can place Secrets on Enemies. Each Secret increases all difficulties, threat values, and retreat values of that Enemy by 1.

Note: Enemies do not automatically attack in the encounter phase. They do not prevent you from performing tasks. They also do not prevent you from leaving a location (unless an Enemy has the Confrontation keyword).