Fetch, Fleinn, Thunderstone And Otherworld

Today, apart from other activities, I have written on paper for a local meeting, not on screen for blog publication. Back to Poul Anderson, Mother Of Kings (New York, 2003):

"...Thorgerd Shrine-bride. Lesser goddess, valkyrie, fetch, whatever she was - yes, through a sign or a dream, a Thraandish lad might well come to believe that such a Being was his heavenly helper, even as a Christian might call upon some one saint."
-Book Four, Chapter XXVIII, p. 391.

"...Being..." is clear from context but is this just Anderson's usage?

"He kept a thunderstone..."
-Chapter XXIX, p. 393.

"'You shall forge me an arrowhead...For a fleinn.'" (ibid.)

I cannot find "fleinn."

"This was a day cool and sweet. Grass glowed newly green. A wind laden with earth-smells as well as sea-tang soughed through pine boughs. Birds filled the blue with wings and cries."
-Chapter XXVIII, p. 387.

I quote this short descriptive passage because it comprises four sentences and appeals to four senses.

"...she saw - half saw - things shine and waver above the haunted mire, glimpses of trees and water and hills nowhere hereabouts, the Otherworld abroad at midday."
-Chapter XXIX, p. 396.

A mirage would indeed look like an intrusive Otherworld.