Pirates Of The Caribbean actor Mackenzie Crook, Hustle's Adrian Lester, Charles Dance and Sarah Parish will join the regular cast when it returns to BBC1 in the autumn.
Crook will guest star in the opening episode as Cedric, a "creepy" character determined to become Arthur's right-hand man.
Lester will play the devious Myror, Cutting It actress Parish a troll masquerading as Lady Catrina, and Dance will guest star as the sinister Witchfinder Aredian.
"With its combination of fantasy, humour and adventure Merlin has firmly established itself as a Saturday evening family fixture. It's great to be working again with the brilliant Shine team who have put together a thrilling second series," BBC executive producer Bethan Jones said.
Johnny Capps, of Shine Television, promised that viewers could expect a "bigger, bolder" second outing.
"We're thrilled to be producing another series of Merlin for the BBC. It's going to be bigger, bolder and more action-packed than ever, with great new villains, white knuckle fight sequences and hilarious comic fun for the whole family to enjoy," he said.