Lake Nostos is an allusion to the Lady of the Lake's lake from the Arthurian Legend.
Before First Curse
Prince Charming travels to this lake to retrieve some of its water for Princess Abigail because her true love, Frederick was accidentally turned into a golden statue by her father, King Midas, whose touch turns anything into gold. Legend has it the waters of Lake Nostos in the Enchanted Forest can restore life to someone who is cursed. Prince Charming successfully gathers the water after killing the guardian of the lake, the Siren, when she tried to lure him to his death. Abigail uses the waters to revive Frederick back to life. ("What Happened to Frederick")
After Prince Charming and Snow White plan to take back the kingdom from King George and the Evil Queen, their journey is sidetracked after Prince Charming's mother, Ruth is severely injured during an attack and struck with a poisoned arrow by one of King George's men. Lancelot, who left the service of King George and joins the party, goes to the lake seeking out its waters to get some for Prince Charming's dying mother. Unfortunately, the lake has all but dried up since the Siren has been killed. Lancelot finds a few drops of the lake's water left, but Ruth is reluctant to drink it after learning the king has cursed Snow White to become barren. Knowing the water can cure the princess, Ruth secretly switches it out to Lancelot. As Ruth is near death, she asks to see her son and his love be married. During the ceremony, Lancelot pours the lake's water into a cup that is thought to have mystical properties of granting eternal life. Snow White drinks it and is healed of her barrenness, though she doesn't yet know. Soon after they are married, Ruth perishes. Afterwards, when Prince Charming attempts to use his mother's old necklace, which has the power to determine the gender of a woman's first-born child, on Snow White, she is shocked to see the pendant works. Only then, she realizes the truth extent of Ruth's sacrifice for her. ("Lady of the Lake")
After First Curse
Emma and Mary Margaret become trapped in the Enchanted Forest after they are accidentally transported there by the use of a hat. They team up with Mulan and Aurora, and for a brief time, Hook, to find a way back to Storybrooke. Emma is given an enchanted compass by a giant that will help them return home. After Aurora is kidnapped by Cora and returns unharmed, they continue on their journey, but become trapped in Rumplestiltskin's old jail cell. They free themselves in time to reach Lake Nostos as Cora uses magic to restore the water of lake, at least partially, and Hook pours the ashes of the magic wardrobe into it to create a portal to Storybrooke. Emma, Mary Margaret, and Mulan stop her and Hook from going through after a battle. Mulan hurries off to restore Aurora with her heart after Hook gives it back. Cora makes an attempt to tear out Emma's heart, but is blasted back by a light coming from inside Emma's body. Then, Emma and Mary Margaret jump into the open portal homeward. Cora and Hook are left in disappointment of their failure, but Hook rejuvenates the shriveled magic bean he got from the giant with the water, and uses it to get both of them to Storybrooke on his ship. ("Queen of Hearts")