“Benight” uses be- in its capacity as a Maker of Verbs with the sense of surround.

• 06.026 so that they often caught people benighted near their gates.

The derivative adjective “benighted”carries the metaphorical connotation of having been morally corrupted.  Tolkien uses it here, however, in its old meaning.  Goblins caught travellers who had become surrounded by night.  The OED tells us that this meaning is obsolete, although we find it perfectly understandable.

“benighted, adj.” OED Online. Oxford University Press, March 2015. Web. 7 May 2015.