conj. when; at the time of, in the event that (used only at the beginning of sentences or phrases)

Ahrk fin zul, rok drey kod, nau tol morokei frod, rul lot Taazokaan motaad voth kein!
And the Voice, he did wield, on that glorious field, when great Tamriel shuddered with war!
(Book, Songs of Skyrim)

Likely related to ruz ‘then’.

See synonym fod ‘when’, which is used more generally. Was likely invented to avoid repetition between frod and fod in the Song of the Dragonborn.