The 'Vilia' in Merlin S4 E2 "The Darkest Hour (pt.2)" & 'Vila/Vilia/Vilya' Slavic fairies/nymphs

Vilia

from merlin wikia - Vilia 
The Vilia are water spirits of the brooks and streams. They appear to be benevolent in nature and they havehealing powers.

Appearances
History
When the veil between the worlds was torn, the balance of the world was upset, and good spirits as well as bad roamed freely. A group of Vilia appeared to Lancelot at a river while he was trying to find a place to shelter him and Merlin from the Dorocha's attacks. They took the form of water bubbles floating in the air, and one of them informed the knight that his and Merlin's help would be needed at the Isle of the Blessed. They magically healed Merlin and promised Lancelot they would protect the two from the Dorocha during the night. In fact, they created a sphere of light that surrounded the lake (The Darkest Hour).  It is possible that Freya, after her death, passed to the Spirit World and became a Vilia.

Appearance in Merlin 

Series 4, Episode 2: The Darkest Hour: Part Two

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My theory about the Vilia / Vila , the between worlds state & Lancelot, by @EmrysDragonlord


            “Vila, Wila, Wili, or Veela, Slavic versions of nymphs, beautiful women who live in meadows, ponds, oceans, trees, and clouds. . . They are female fairy-like spirits who live in the wilderness and sometimes in the clouds. . .Believed to be the spirits of women who had been frivolous in their lifetimes, they now float between here and the afterlife. . . They have healing and prophetic powers and are sometimes willing to help human beings.” –wikipedia (see smaller typeface below)

            

            "The voices of the Vilas are as beautiful as the rest of them, and one who hears them loses all thoughts of food, drink or sleep, sometimes for days." In S4E2 when the Vilia tell Lancelot that he is tired and must rest, Lancelot insists that he “must find shelter,” (and in fact did not appear tired) after the Vilia began to heal Merlin. Perhaps the Vilia’s inadvertent magic which “erases all thoughts of food, drink, or sleep,” had some effect on Lancelot, though in this case the Vila had not intended Lancelot to succumb to it. The Vilia, being fickle in general, often leading men to dance with them until they drop or die terrible deaths should the Vila fall in love with them, were not up to their usual mischievousness, perhaps due to special a circumstances in this episode's story.  The tear between the worlds created by Morgana was apparently a terrible thing to the Vilia as well, it was their main concern at the time they met Lancelot.  Their motives for helping the pair of travelers were driven by their desire to repair the tear.

            

            In response to Lancelot’s insistence that he “must find shelter,” the Vilia tell him that they would protect Merlin and Lancelot through the night, and created a bubble of light around them.  Lancelot did not protest or mistrust the Vilia; instead he only smiled and gazed in wonder at the magical force field surrounding him. Because of the docile and unsuspecting way Lancelot behaves in the scene with the Vilia, it is probable that the Vilia did indeed use some type of magic to convince Lancelot to trust them without much protest.

            

            “The Vila are cursed never to find their true love. If they do, that love will die a terrible death.”  -wikepedia (see smaller typeface below)  The mythological definition of the Vila’s state of existence as floating “between here and the afterlife,” and the tearing of the “veil between the worlds of the living and the dead has created a new world” -(the Cailleach /the gatekeeper) means that the world that was created when Morgana tore the veil was an extension of the world of the Vilia into the natural world.  If the voices/spirits of the dead, the “Doraka” can come into the world of the living, or some form of that world without becoming “alive” or “living creatures,” it follows that a living person who walks through the veil to the “Doraka’s” side, will not actually “die” or make that person one of the “dead.” Therefore, I believe Lancelot is not actually dead; whether or not my theory about the physics of this mythical world is something the writers of Merlin share is another story… I also just really don’t want Lancelot to be dead for good, but remember when the producers said “Nobody really ‘dies’ in Merlin.” That’s a good sign!

            

            If Lancelot is not dead, he's floating between here & the afterlife (just like the Vilia).  It would not be unheard of in the world of Merlin, that he may even be in the water with them; as per what happened to Freya aka. The Lady of the Lake.  So if Lancelot does end up coming back, I'm also guessing the Vilia will help him do this.  If not, there is at least a strong possibility he'll come back from the water & maybe it will be Merlin's true love Freya, the Lady of the Lake that plays some part.  Otherwise, assuming his return; water or no water, it will have to do with magic and a place between life & death.  I think the only people / creatures from Merlin we know of so far, that might have the ability to make him come back to the world of the living whether they'd like to help or not are: 

1) The Vilia   
2) Freya aka. The Lady of the Lake 
3) Merlin
4) Morgana
5) A Sidhe  [see S1E7 "The Gates of Avalon" & S3E5 "The Changeling"]

animated gif by jantojones

Reference: 

Vila from wikipedia Slavic fairies

The Vila, Wila, Wili, or Veela are the Slavic versions of nymphs, who have power over storms, which they delight in sending down on lonely travelers. They live in meadows, ponds, oceans, trees, and clouds (cf. Leimakids, Limnades, Oceanids, Dryads, Nephele). They can appear as swans, horses, wolves, or beautiful women.

Ravijojla, a vila of the Serbian mythology, on a painting inspired by the Serbian epic poem "Marko Kraljević and the Vila".

In Polish mythology, the Wiła (pronounced [ˈviwa]), and in South-Slavic mythology the Vila ([ˈviːla]), are believed to be female fairy-like spirits who live in the wilderness and sometimes in the clouds. They were believed to be the spirits of women who had been frivolous in their lifetimes and now floated between here and the afterlife. They sometimes appear as swans, snakes, horses, falcons, or wolves that they can shapeshiftinto, but usually they appear as beautiful maidens, naked or dressed in white with long flowing hair.

It is said that if even one of their hairs is plucked, the Wila will die, or be forced to change back to her true shape. A human may gain the control of a Wila by stealing feathers from her wings. Once she gets them back, however, she will disappear. (Compare Swan maiden andSelkie.)

The voices of the Wilas are as beautiful as the rest of them, and one who hears them loses all thoughts of food, drink or sleep, sometimes for days. Despite their feminine charms, however, the Wila are fierce warriors. The earth is said to shake when they do battle. They have healing and prophetic powers and are sometimes willing to help human beings. At other times they lure young men to dance with them, which according to their mood can be a very good or very bad thing for the man. They ride on horses or deer when they hunt with their bows and arrows and will kill any man who defies them or breaks his word. Fairy rings of deep thick grass are left where they have danced; these should never be trodden upon, as this brings bad luck.

Offerings for Wila consist of round cakes, ribbons, fresh fruits and vegetables or flowers left at sacred trees and wells and at fairy caves.

In Croatian folklore, the mythical Velebit mountain range is famous for its fairies, the most celebrated called Velebitska Vila or Vila Velebita("The Fairy of Velebit"). The Vila is described as being a good spirit, and is the patron of the Velebit mountain range, whose significance in Croatian culture has led to tales and songs of the Vila, the most popular one created in the 19th century titled Vila Velebita, which is still popular today.

Named vilas in the Serbian mythology are: Andresila, Andjelija, Angelina, Djurdja, Janja, Janjojka, Jelka, Jerina, Jerisavlja, Jovanka, Katarina, Kosa, Mandalina, Nadanojla and Ravijojla.^[1] Ravijojla is the best known of them,^[1] connected to Prince Marko,^[2] while Jerisavlja is considered to be their leader.^[3]

Western European References

Vilia

In a love song titled Vilia, from "The Merry Widow" by Lehar and Ross, a hunter pines for Vilia, "the witch of the wood". Judika Illes also mentions the Vilia in her book, The Encyclopedia of 5000 Spells.

In some tales, the reason for abandoning their loves is a sad one. The Vilya are cursed never to find their true love. If they do, that love will die a terrible death.