77130: Vermin: Beetle Swarm, by Bob Olley.
I got them amongst the first Bones Kickstarter, and in truth they're not the sort of thing I'd normally spend any actual money on. You'd get much the same effect by printing pictures of swarming beetles and sticking them to tiddlywinks or something. However, I've got them, so I thought I might as well make use of them.
I haven't yet looked at the vermin swarms in the new Monster Manual, so I don't know how frightening they're likely to be. I'm imagining something like the swarms of carnivorous scarabs in The Mummy, which could be fun. Though I may need a few more beetle-swarm-markers.