The Sirens are an adventure map structure in The Price of Loyalty, Heroes III, and Heroes V.
They can be found at sea, and if anyone visits them, they will kill 30% of that hero's army, but give them experience based on the hit points of the creatures killed - stronger creatures and larger armies give more experience than smaller, weaker forces.
While the extra experience is certainly useful, it's not always worth the cost of the creatures. It's definitely useful in campaign if the enemy has been all but defeated, and the hero is still missing a lot of experience to reach the level cap. It can also be used to get rid of unwanted creatures, but other than that, the reward is rarely worth the cost.
