If you play with Zerg against a Terran the importance of the defiler will be seen at the late stages of the game.The most important Defiler ability is Dark Swarm, especially against ranged units like marines, dragoons,or may be tanks. The other original ability is its consume ability which gives this unit a theorotically infinite energy if you always have a sacrificed unit near it.But remember an ultralisk or a zergling gives the same amount of energy so be careful.Never sacrifice your elite units to obtain extra energy for your defiler.The third ability is plague which is very harmful to the Terran because Terran buildings have  self decreasing energy from a critical point so the plague reduces the energy level to the critical point.You must also remember that always burrow your defiler after your mana attack to the enemy so you can protect your defiler easily.The defiler technology gives Zerg swarm a very high strike chance with few losses so if you play against Zerg you must stop him before he develops defiler technology.They are really dangerous and will absolutely change the game’s destiny.