During the lifetime of the children of Bjorn skafinn, the road between Njardvik and Borgarfiord, over the so-called Njardvik-scree, fell into disuse.  The reason for this was that an evil being, whose upper part was in the shape of a man and the lower of a beast, would lie in wait for men on the road when nights began to get dark, killing many, so that it became impossible to go that way.  The monster hid in a gully on the Njardvik side of the Scree, since known as Naddagil; for on autumn and winter evenings, a noise of grinding and clashing stones could be heard all the way to Njardvik.  For this reason also the monster was called Naddi, which means the gnasher.
Once, when autumn was far advanced, Jon Bjarnarson arrived late in the evening at Snotrunes in Borgarfiord and intended to go on to Njardvik.  Since it was after sunset, though, men urged him not to take such a risk and travel over the Scree so late.  However, he paid no heed to their warnings, saying that he would come to no harm, for there was not the least danger, and so he set off on his way.  But when he got to the gully, the monster appeared and attacked him.

Their struggle was long and fierce.  They fought all the way to the bank known as Krossjadar.  Here, said Jon, the monster tore itself away from him and crawled into the sea.  A cross was set up at the place afterwards, and stands there still, with an inscription in Latin bidding travellers kneel down and say a paternoster, and this custom was observed for many years.

Jon reached Njardvik much the worse for wear, black and blue all over, and kept to his bed for a month after which he got up.

He maintained that the monster had come up out of the sea, since it returned thither at the last.  However, it was never seen again after their encounter.
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