An Elvish Avenger is an elvish unit and a level 3 unit. It is the highest form for this line of upgrade. It is equipped with a skill called Ambush which activated when the unit step into any forest. It is one of the best level 3 units in the game and one of the rare mixed fighters, the other famous one being the human Ranger.
The curious name of the Elvish 'Avengers' comes from a tactic often employed by these master rangers. The enemy is allowed to break through a feint defense, and when the vulnerabe troops behind the front lines follow, these Archers break cover and attack, cutting the supply line and surrounding the enemy in one fell stroke.
This has, at times, been interpreted as a form of vengeance for their brethren lost earlier in the battle. Although not so base in design, it is not at all inaccurate.