Energy

Energy is the Assassin's equivalent of the caster's Mana and the warrior's Rage bar. A certain amount of Energy is required by most Assassin abilities to be used.

By default the Energy bar is full, with 100 points of Energy.

Energy Regeneration
Energy regenerates at a fixed rate of 10 energy every second, in and out of combat

Another way to obtain Energy is by using Basic attacks (Standard Keybind being Spacebar) which will give 5 Energy per attack.