Rusalka - Terrible Fairies
Rusalka are Slavic fairies (Fey), that are best avoided. Similar to the Germanic Nixies, and Mermaids in some tales, they are water spirits who are female & usually malicious. This can range from turning water bad to drowning innocents. A few are seen as benevolent, though with them being Fey they are naturally fickle, and so can be either. Originally they are thought to help with crops by coming out and leaving moisture on crops to help them grow, at least in the spring. They can appear as small girls with pale skin & green hair, or young women with red or blonde hair, while a few tied to forest waterways have black hair.