> "Tröll (neut nom plural of troll, vowel shift in plural cf land,

lönd) hafi þína vini (acc plural)," segir hún.

> '(May) trolls have (take) your friends,' she says.

I guess it is plural from the context, but the singular is 'tröll'
too. Another spelling is 'troll', also identical in the singualar and
plural. Presumably 'troll' is the older form, cf. 'trylla' "to
enchant, turn one into a troll; call one a troll" = Da. trylde, Sw.
trolla. Do any other words undergo this vowel change, or is 'troll' >
'tröll' unique?

Incidentally, the OED says that the word came into English from only
in the 19th c., but that in Orkney and Shetland, "it has survived from
the Norse dialect formerly spoken there." It backs this up with some
suitably sinister quotes from 17th c. witch trials:

1616 Dittay Sheriff Court Shetland 2 Oct. (Jam. s.v. Trow), The said
Catherine for airt and pairt of witchcraft and sorcerie, in hanting
and seeing the Trollis ryse out of the kyrk yeard of Hildiswick.

1640 Orkney Witch Trial in Abbotsford Cl. Misc. I. 167 ye ansuered hir
againe, that it was but the Trow that haid gripped her.