Final answer:
The character in the play Medea who appears only once and describes the death scenes of Glauce and Creon is the Messenger.
Step-by-step explanation:
In the play Medea, the character who appears only once and relates in gruesome, vivid detail the death scenes of Glauce and Creon is the Messenger. In a monologue, the Messenger describes how Glauce died by falling on a keen-edged sword beside the altar and how her dying words included cursing her killer. The Messenger also describes how Creon's wife, Eurydice, is seen approaching the palace gates, indicating that her son's death has been discovered.