Reveals Weakness

Now it is told that Guđrún had enchanted the armor of her sons so that
iron could not pierce it, and she told them not to injure any stones or
other big things and that if they did, they would no longer be shielded
from all harm.

[...]

They went on to the realm of King Jormunrekk, and they went
before him and attacked him. Hamđir cut off both his hands, and Sorli
both his feet.

Then Hamđir said, “His head would have come off, too, if our
brother Erp had lived, but we killed him on the road and we realized
our error too late,” as the poem says:

“Jormunrekk would lose his head,
if only Erp still lived,
our bold brother,
the one we killed on the road.”

And when they had done this, they had violated the command of
their mother, because they had injured stone. Now Jormunrekk’s men
attacked them, and they defended themselves well and boldly and
injured many men, and iron would not harm them.

But then an old man came, very tall and with only one eye, and he
said, “You are not wise if you don’t know how to kill these men.”

King Jormunrekk said, “Advise us if you know how.”

The man said, “Kill them with stones.”

And this was done. Stones were thrown at them from every
direction, and they were killed.


- Volsunga Saga, Chapter 42