from the Synaxarion:
Called the Self-Sacrificed by the people, he was descended from the Bagratid dynasty and was the son of the emperor David V (+ 1269). The Emperor Demetrios exerted much effort in the enlightening and peaceful prospering of his land. During his reign were annexed the Armenian provinces adjacent to it, which aroused the displeasure of neighboring Persia. But thanks to the wise actions of Saint Demetrios II, rendered in a series of services to the Persian sultan Akhmed, a clash with Persia was successfully averted over the course of some several years.
The new Persian sultan, Argun, however, heeding the complaints of his court Jewish physician, conceived a strong hatred within him towards the Orthodox Emperor Demetrios, and he set out with a large army to the borders of Georgia. Sultan Argun set up his encampment on the Mugan plain. Holy Emperor Demetrios, wanting to save his land from being overrun with devastation, came himself into the camp of the enemy and attempted to assure him of his peaceful intentions.
The sultan in an uncontrollable rage offered the saint a choice -- death or the despoiling of Iveria. Saint Demetrios answered the tyrant: "I shalt sacrifice my life for the welfare of my subjects". Saint Demetrios was executed (+ 1289). The Georgian and Armenian historians relate that several hours after the martyr's end of Saint Demetrios, the sun suddenly darkened and terror overcame sultan Argun and his army. The Persians in fear left Georgia, without wreaking ruin upon it.
"The memory of holy emperor Demetrios, named the Self-Sacrificed by the Iverians, is revered as holy in the land, which he did save from the tyrant by the sacrifice of his own life".