Cavity Spider

When Cavity Spider attacks, it must be blocked
by the creature with the least toughness.
(others can block it as well)
"It is always surprising to see how small a crack they
can hide in. They prefer let most of a group walk by,
the leap out and strike at the tail member. Worst
thing is, when the last person in line just disappears,
you never really know what happened to them or
when."