In series 4, Uther is wounded during an assassination attempt on Arthur. Merlin tries to heal him but due to Morgana's meddling, the spell instead kills him. In the television series Camelot, Uther is poisoned by his daughter Morgana in the first episode.

What episode did Uther die?

Merlin series 5 episode 3 review: The Death Song Of Uther Pendragon.

How did King Uther die in Merlin?

He later died at her hand after having been mortally wounded by an assassin, with Merlin's healing spell backfired by Morgana's enchantment. After his death, he became the father-in-law of Arthur's wife, Gwen.