This explains a few things, the invisible beasts charging out of the trees. Because Loki has been idly looking for them, and up until now encountered few. Perhaps they were avoiding him, but it's also possible he's wandered past them while they were invisible, which is an irritating thought.

He remembers what the child that was attacked said, back at the beginning of the winter, and how Harrowheart said he'd killed one of the monsters without actually seeing it. Does this mean they can have no clear idea of numbers? The whole lot of them could be walking into an impossible ambush.

He almost makes a move to intervene on Sif's behalf, but he's not shocked when she displays how little she needs his concern by turning and blocking her attacker. Good. Now the thing he probably should be worried about is getting mistaken for one of the invisible foes, rather than the invisible sort-of-ally he actually is. He darts toward the incoming invisible beasts, his own knives out and cutting indiscriminately at whatever gets within his reach. Perhaps he can use the element of surprise to draw blood--or vapor, whatever they have in them--and make them a bit easier for the others to see.