Episode 3 of “Once Upon a Time” Season 5 dropped a bombshell Sunday night -- King Arthur (Liam Garrigan) is not who he says he is. Lancelot (Sinqua Walls), who was believed to be dead, snuck into Camelot to warn Mary Margaret (Ginnifer Goodwin) and the others they cannot trust the king. But is Lancelot’s warning too late?

The promo video for episode 4, titled “The Broken Kingdom,” reveals King Arthur will reveal his true self to the Storybrooke heroes. And unfortunately it will put Emma’s (Jennifer Morrison) life at risk.

Now that David (Josh Dallas) is a Knight of the Round Table, King Arthur will confide in him that Excalibur is not whole. He’s been searching for the missing piece, which has been turned into the Dark One’s dagger. King Arthur further manipulates David into believing if they hand over the dagger he could use it to help Emma. That’s when Mary Margaret comes clean about Lancelot’s visit.

“We can’t trust Arthur,” she tells her husband.

Of course Excalibur combined with the Dark One’s dagger can defeat the darkness. But that also means killing Emma -- something the Storybrooke heroes are not willing to let happen. Mary Margaret, David and Lancelot team up together to fight King Arthur for control of the dagger, but, meanwhile, Emma will be fighting her own inner demons.

“You’re the Savior,” Emma’s vision of Rumpelstiltskin (Robert Carlyle) tells her. “Save yourself -- before it’s too late.”

Although Emma has been trying not to use her dark magic, it sounds like the circumstances might force her to do something she regrets. Could that be what triggers the memory erasing curse? “Once Upon a Time” fans can find out when episode 4 airs on ABC on Sunday, Oct. 18, at 8 p.m. ET.