Vicious carnivorous frogs that attack in swarms with flesh-shearing teeth

Grimfrogs are vicious carnivorous frogs. An individual grimfrog looks like an ordinary small green frog, at least until it opens its mouth and shows off its fearsome array of interlocking flesh-shearing teeth. However, the real danger of grimfrogs is that they attack in swarms. Because of their small size and innocuous appearance, grimfrogs surprise on 1-3 on 1d6 even if they have been noticed by their potential victims (unless such victims are already familiar with grimfrogs). An ordinary swarm of grimfrogs consists of one individual per hit point. They attack by jumping onto a victim and biting any accessible flesh. If a grimfrog swarm scores a hit, it does 1 point of damage plus 1 additional point for each point the attack roll exceeds the victim's armor class (but never more than the number of surviving grimfrogs in the swarm). A swarm may split into two or more swarms of roughly-equal numbers, dividing the swarm's hit dice between them; no smaller swarm will be formed that has less than 10 hit points, so for example a swarm having 20 hit points can divide into no more than two smaller swarms. A swarm will usually split in this way if there are several potential victims close