Unexpectedly, they sang

transitive or intransitive

It can be either

Explanation:

In the first, “sang” is a transitive verb with “Happy Birthday” as its object. But “sang” has no object in the second sentence (i.e., we don't specify what we were singing). As such, “sang” can be either transitive or intransitive depending on how we use it.

moon-letters were a method of writing invented by the dwarves. moon-letters were ordinary runes, but written in such a way that they could only be read when the light of the moon shone behind the writing. some moon-letters could be read by any moonlight, but it was possible to make the writing even more secret. more sophisticated moon-letters could only be read by the light of the same shape of moon, or at the same time of year, as when they were written. hope this !