Undines  (also known as Ondine) are water spirits that work for the primal god Necksa. The Ghostbusters attempted to trap some of them, only to realize that Peter Venkman forgot the Ghost Traps. Necksa is very passionate of the Undines, as he refers to them as his children. 
Undines are water elementals and traditionally benevolent spirits.  They are ruled by the primal god, Necksa. In the 20th century, in the midst of a 12 day heatwave, Necksa awoke and was angered by the pollution in New York and sent the Undines to take over the city. They used all water sources as a medium for travel and haunted the water supply of all five Boroughs of New York.
The Ghostbusters were called into action and were able to confine several in the Proton Streams but Necksa was powerful enough to safely recall them. Based on their chanting of the word "Necksa," Egon Spengler located Necksa's entry in Tobin's Spirit Guide. After Peter Venkman made a bargain with Necksa, it is assumed the Undines left New York City with their master.
According to MythologyEdit
Undines are female water spirits who are interchangeably known as water spirits, nymphs, fairies, and even wraiths.  A Renaissance scientist named Paracelsus theorized Undines inhabit forest pools and waterfalls, their beautiful voices can sometimes be heard, and they must marry a man and bear him a child in order to earn themselves a soul. 
Undines can travel through sources of water, fly, and instead of sliming people, they drench them in water.
- ↑ Ray Stantz (2009). The Real Ghostbusters - "The Devil in the Deep" (1987) (DVD ts. 5:41-5:43). Time Life Entertainment. Ray says: "My guess is they're Undines. Water Elementals."
- ↑ Necksa (2009).The Real Ghostbusters- "The Devil in the Deep" (1987) (DVD ts. 11:13-11:17). Time Life Entertainment.
- ↑ Egon Spengler (2009). The Real Ghostbusters - "The Devil in the Deep" (1987) (DVD ts. 5:46-5:50). Time Life Entertainment. Egon says: "Undines are traditionally benevolent spirits and these are expressing highly malevolent traits."
- ↑ http://en.wikipedia.org/wiki/Ondine_%28mythology%29
- ↑ http://www.pantheon.org/articles/u/undine.html