me'Grmat XVIII was a male al'Hmatti who served as emperor of the planet taD in the late 23rd century and/or the early 24th century. He died after a long illness, and was succeeded by te'Osbron, who became me'Grmat XIX. (TNG novel: Diplomatic Implausibility)

The period of me'Grmat XVIII's reign is supposition, given the lack of reference points within the story.