Rusalkas
Rusalkas are water nymphs of Russian mythology, beings that bear a certain resemblance to sirens. Rusalkas are said to be the spirits of young women who perished by drowning. They thus inhabit rivers and lakes, and on moonlit nights they sit on the banks to comb their green or golden hair while singing and dancing. Rusalkas have a human appearance but possess skin so white it can be translucent. They may also have a fish tail or transform into a toad, a frog, or a fish.
Rusalkas are lonely creatures, and for this reason they occasionally abduct humans to take them to their crystal palaces at the bottom of the waters. To protect oneself from such abductions, one must make the sign of the cross or wear a cross around the neck.