|Whirlpool as seen on the Adventure Map.|
Whirlpool is an Adventure Map object, that can only be located on water terrain. It can only be entered with a hero traveling on boat, and it instantly transfers the hero (and the boat) to another randomly chosen whirlpool. If there is only one whirlpool in the map, it does not transfer hero anywhere but it will still cause the hero to lose part of their army.
When a hero enters the Whirlpool, he loses 50% rounded down of the creatures in his army's weakest stack. The weakness is determined by total health of the stacks. With Sea Captain's Hat or Admiral's Hat (only in Shadow of Death), a hero can enter Whirlpools without losing creatures. To avoid losses, a player should divide one of the most disposable creatures to an individual slot, which naturally becomes the weakest stack and is therefore lost during the teleportation. If a hero with only one creature enters a whirlpool, the creature (and the hero) will not be lost, making whirlpools "safe" for scout heroes (ignoring that you usually can't know where the hero will end up).
Whirlbools have six squares which can be entered or transfered to. This can easily cause a situation, where the boat cannot move outside the Whirlpool area after the teleportation. Note that the space key can used in the expansions to re-enter the current square to avoid unnecessary movement.