Sirens are a female cross between mermaids and vampires. According to the concept art, Sirens have extremely long hair and they are nude with a blue glow. Originally, Sirens were going to be part of Fable II but got left out. Their existence in the Fable world is suggested in the song, "Down by the Reeds" that Hammer sings outside of the Wellspring Cave in Fable II. Owners of the Limited Collectors Edition Fable II Art Book can see concept art of the Sirens on page 76.
Sirens make an appearance in Fable: Edge of the World as well, posing a threat to travellers in Samarkand. They remain a persistent danger to the people of that mostly-desert land, inhabiting the few but vital bodies of water and daring the locals to come gather it. One more prominent such lake was Sky Blue Lake.
It was here that the Royal Army of Albion was lead by the mighty Hero of Brightwall as they marched through the desert, driven to stop by their guide, Shan, for a respite before the march to Sweetwater Trees. However, while a number of soldiers, including Ben Finn, fell to the charm of the sirens, Albion's mighty Hero King realized that while their were a series of nomad tents strewn about the place, their occupants were all missing. Using his command of Will, he quieted the song of the sirens long enough for both Ben Finn and his fellow soldiers to break free from the spell before marching on.