Merlin: The Return (2000) on Netflix

Plot summary

Mordred and his mother Morgana were banished to another world, where Merlin's magic has held them for 1,500 years. The knights of King Arthur's court lie in a suspended sleep, kept from harm by a spell that draws on the ancient power of Stonehenge. In the present day, the scientist Dr. Joan Maxwell finds a gateway that joins her world to the one that holds Mordred. With the way open, King Arthur is woken from his long sleep and joins Merlin and Lancelot. They set out to stop Mordred from returning, and Mordred has a scheme to enslave Arthur's world.