The draug is a mythical creature, straight from ancient legends of heroes and epic deeds. When the hero enters the burning hells to rescue his beloved, or when he has to avenge his father's death, the draug is often his opponent. Why are poets so keen to cast this monster as the arch-enemy? Well, the draug is a wraith, so it fits any dark story featuring a curse or vengeance from the beyond. There's no telling how what it actually looks like, so its terrifying visage can be described in many ways without risking accusation of confabulation. Furthermore, it is a powerful creature, a prince of the damned, so it makes an ideal villain.
As an arch-wraith, the draug never stoops to doing anything with its own hands. It has lackeys for that, always wraiths, revenants or other restless spirits. Having been a king or a commander in life, the draug retains its charisma in death, and its deathly subordinates always blindly obey its orders. Thus one needs to exterminate all manner servants on the way to its underground palace, wilderness keep or other foreboding abode. Only then can one face the draug itself. That's literary tradition for you.
The draug can described in various ways, as has been said, but is always a lethal foe. Forget its huge strength, invulnerability to pain, fearlessness and bloodlust. One cannot just defeat a draug using conventional means. As with every wraith, there is some tragic event connected to it, forcing the creature to remain among the living. The draug is untouchable, unreachable for anyone who is not part of that story. That is why the mythical hero has to get involved in various brawls and pass through many trials. By overcoming these obstacles, he enters the draug's world and becomes worthy of facing the monstrosity.
The wraiths' leader is so powerful that it mocks simple witchers1 tricks. The one sure thing in fighting the draug, it must be said, is a silver sword.