--- In norse_course@yahoogroups.com, "llama_nom" <600cell@...> wrote:

>
> I suppose just a special instance of its use as a relative pronoun.

In such instances, rather than referring back to a noun in the main
clause, 'er' represents a clausal subject, whether prefigured by 'þat'
or not.

In some examples anyway. Others don't necessarily fit that model:

Boet við Hrímgerði, er þú lézt höggvinn Hata.
Compensate H. for having killed Hati.

Iðrumk ek mjök, er honum er gipt dóttir Magnús konungs.
I regret greatly that the daughter of King Magnus has been married to him.