Mortaeus Flower

"It says here that someone poisoned by the Mortaeus can only be saved by a potion made from the leaf of the very same flower."

- Gaius to Arthur and Gwen

Mortaeus Flowers are rare plants that can only be found in the caves beneath the Forest of Balor. Their petals are poisonous, and they grow on the root of the Mortaeus tree.

History
"Few who have crossed the Mountains of Isgaard in search of the Mortaeus flower have made it back alive."

- Gaius to Arthur

The High Priestess Nimueh used an enchanted Mortaeus petal to poison Merlin. She hid it inside a chalice that she later tricked Merlin into drinking from during a banquet in Camelot.

After Gaius determined the nature of the poison, Arthur traveled to the caves beneath the Forest of Balor to find the flower and procure its leaves.

He was arrested upon his return for disobeying the King, but Gwen managed to retrieve the flower from the dungeons and deliver it to Gaius in time for him to brew an antidote (The Poisoned Chalice).

Uses
"The Mortaeus induces a slow and painful death."

- Gaius to Arthur

The poisonous petals of the Mortaeus flower were capable of killing a human in four or five days. Merlin fell into a coma after ingesting the poison and quickly developed a high fever, respiratory distress, and delirium. He later developed a rash on his arm when the poison reached its final stage, at which point death could be expected within two days.

The effects of the poison could be accelerated by an enchantment. Nimueh enchanted the petal she used to poison Merlin by swirling it in her scrying fount while chanting a spell.

The only known cure for the Mortaeus poison was a potion made from the leaves of the same flower. If the poison was created with magic, then the antidote also needed to be made with magic to be effective (The Poisoned Chalice).

Trivia

 * The plant used to depict the Mortaeus flower is a Masdevallia Pleurothallidinae, a non-poisonous hybrid orchid originating from Central and South America.