. . . He would keep the pledgeand the vow of friendship as long as he lives,that which in former days you two often uttered. (52-54)
What the lord wishes to signify with these runes is his commitment to keep the previously spoken vows. He swears an oath in order to reaffirm the previous one. Basically, he's saying that he's a stand-up guy, one who keeps his promises, and he's so serious about this that he's willing to make another vow to prove it.