Draugr are said to be able to shapeshift. Most famously, in the Laxdeala Saga, into the shape of a seal. This is referenced when the men believe they hear seals, only to encounter the Draugr.
A draugr is a corporeal (meaning it has a body) undead spirit from Old Norse mythology. They typically haunt the area near the cairn they were originally buried in. Draugr resemble a reanimated, decaying corpse and are noted for their disgusting stench of rotting meat. Aside from inhuman strength, Draugr are able to visit the dreams of the living (often leaving behind a tangible gift or physical sign found upon waking to assure the victim their dream was real) and can kill victims with bad luck or curses.