Well, the poem evolves around settings such as twilight, dusk, fire, plants, and birds. I would personally say the meaning behind it could vary between physically dark occurrences that aren't viewed as a true form! ( if ya get what I'm sayin)
Answer: n this line, Donalbain is saying that he and his brother Malcolm are not safe if they stay where they are. That the men who smile at them are actually concealing daggers, wanting their blood.
Explanation: