Arthur (Bradley James) will be "forced to confront magic" in the new episodes, Morgan told SciFiNow.
"That could change everything, for Merlin and for everyone," said the Irish actor. "[We have the] most important scenes there has ever been in terms of Merlin's magic."
He continued: "It's definitely the most ambitious series yet. They've pushed our characters into manhood, and this is the Merlin I've been waiting for."
Morgan added that his wizard character was last seen "tipping over into a darker side" at the conclusion of the BBC drama's third run.
"I think that's a reference for me for the rest of this season," he explained. "I think he is so confident in himself now, and in who he is and what he is destined to do.
"He has a resilience and power that drives him for this season. He gives as good as he gets."
The fifth series of Merlin is currently shooting and will air on BBC One in late 2012.
