Colin Morgan (Merlin), Katie McGrath (Morgana) and Angel Coulby (Gwen) spoke to Digital Spy about the new episodes, Gwen's reign as Queen and the return of the dragons.
Morgan told Digital Spy that his young wizard character is in a "ruthless mode" in the next series.
"Merlin is becoming more ruthless, because he is becoming more solid in his beliefs," said the 26-year-old. "He's seeing things in a whole new way."
Mordred - now played by Alexander Vlahos - will also return to the series, with McGrath suggesting that he and Morgana have an "unbreakable" bond.
"You can't tell the story of Morgana without Mordred," she explained. "They are interlinked, they are entwined, and from the very first episode [of series five], you see that."
Morgan added that the reappearance of Mordred is "the biggest threat there's ever been" to Camelot and the reign of Arthur (Bradley James).
"Anyone who knows the Arthurian legend knows what Mordred is destined to do... so that is the biggest threat, that is the biggest force," he hinted.
> 'Merlin' star Colin Morgan on series five: 'It's about the legends'
Merlin exec Julian Murphy also teased the possibility of a sixth series of the BBC fantasy drama, saying that he is "not ruling it out".
"It's a possibility - we don't know the answer yet," he said. "There's conversations going on, but I can't really say any more than that."
Merlin series five will air on BBC One in late 2012.
> 'Merlin' series five trailer released by BBC - watch
> 'Game of Thrones' Samwell Tarly actor John Bradley cast in 'Merlin'