WARNING: The following contains spoilers for The Third Day episode "Sunday -- The Ghost," now airing on HBO.

While The Third Day didn't give Epona (Jessie Ross) much screen time in its first three episodes, there was clearly something very important about her and a reason why she had an effect on Sam (Jude Law). A number of things are keeping him on Osea Island, as he witnesses fascinating yet horrific pagan rituals and Celtic sacrifices being used to revitalize the land, but his affair with Jess (Katherine Waterston) and concern that Epona is being abused are two prime factors. However, "Sunday -- The Ghost" finally reveals the shocking truth about the teenage girl and her destiny, which is to make the land and sea bountiful again.

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The Third Day kicked off with Epona trying to hang herself off the mainland and Sam rescuing her. He swore he saw his son Nathan as well at the scene, so this inspired him to bring Epona to her family on the island. After seeing her dad's anger, Sam left Epona with the innkeepers, the Martins, but he still feared for her safety. Given that Nathan was killed off the mainland years ago, he's driven by grief and didn't want Epona to relapse into depression again.

However, "Sunday -- The Ghost" finds Sam hiding from Larry, who wants to kill him, and the Martins confess Epona's hanging is more than just her being sad: she willingly did it as a sacrifice to the Celtic gods. The islanders wanted Sam to come home to claim his throne as Osea's "Father," so they kidnapped Nathan years ago and swapped Epona's little brother's body for his. Nathan's been kept on the island to be molded into a future leader, but with Sam's granduncle and not his grandfather (the rightful heir who fled) at the helm, the island is rejecting the people.

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Epona tried to offer herself as a sacrifice, but there was seemingly some divine intervention in Nathan being there at the same time Sam Showed up. Apparently, Osea's spirit, as it's considered the heart and soul of the world, wanted Sam back home and brought everyone together, manipulating events in such away the Martins -- who arranged Nathan's abduction -- couldn't predict. Sam eventually loses it after hearing about the botched kidnapping, but as he sneaks out, fed up with the island and hating his birthright, Nathan, who's confirmed as real and no longer a ghost, lures him to a church where Sam finds pagan symbols indicating he's meant to lead through human sacrifices.

Sam is disgusted. However, he eventually finding Epona on an altar, gutted and dead. Mrs. Martin approaches with a shotgun to confirm Epona completed her suicide. It's a ceremony where she once more offered herself to the island, thinking Sam would renounce his destiny. She also liked him as a dad, so it appears Epona wanted the gods to be satisfied with her rather than have Sam suffer through a leadership position he doesn't want. Sam is devastated, but Mrs. Martin admits Epona won't be enough and starts relentlessly hunting Sam with a shotgun.

It brings full-circle the theme for Sam of failed parenting, as Epona had become a daughter to him and, once more, he failed. But there's not really much he could have done, since the island was using Nathan and her as pawns, knowing every year Sam came to the mainland at the site he thought Nathan died. What makes this harder is Epona's dad is the one caring for Nathan, making him a substitute for his own dead child, which explains why he hated seeing Sam on Osea. But he now tries to help a fleeing Sam, telling him where Nathan is, and how to get off Osea, understanding no father should have to endure what they both did.

Starring Jude Law, Naomie Harris, Katherine Waterston, Nico Parker, Charlotte Gairdner-Mihell, Paddy Considine and Emily Watson, The Third Day airs at 9 p.m. ET/PT on HBO.

KEEP READING: The Third Day: Osea's HORRIFIC History Has Finally Been Revealed