The whirlpool is an adventure map object in the Heroes of Might and Magic series that can be used to travel to another part of the map. The hero can sail into one whirlpool, and will instantly emerge from another, usually at the cost of some of their creatures.
In Heroes I and Heroes II, the whirlpool has 1/3 chance to kill 50% of the creatures in the hero's lowest-tier stack. In Heroes III and Heroes V, the whirlpool WILL kill 50% of the creatures in the hero's lowest-tier stack.
If the hero has more than one stack of the same tier, only the first one will be damaged. It's therefore wise to split a single creature off from the main stack and place it first in the hero's army, which will allow the hero to travel without creature cost.