> sá hafði lýst sökinni er sannir lagalestir voru á og sjálfur var

útlagi.

"he who gave notice of the action (publicly announced that the case
was being brought) was guilty of proven breaches of law and [he] was
himself an outlaw."

Or more literally: "HE gave notice of the action ON WHOM were true
breaches-of-law and [he] was himself an outlaw."

Or even more literally: "HE gave notice of the action WHOM true
breaches-of-law were ON, and [he] was himself an outlaw."

sá...er "he who(m)"
sé...er sannir lagalestir voru á "he on whom (on whose conscience)
were undeniable offences"

lýsa (lýsta, lýstr). (Z4) proclaim, declare, make known.

laga-löstr (pl. laga-lestir). breach of law, legal evasion/fault/offence.

sannr. true, proven, certain, clear.