Here is a 14th century fresco from Vysokie Dechani, in Serbia:
The visible inscription says only МОЛЕНИЕ/MOLENIE — “Prayer.” and below that we see the common title inscription identifying the woman as ΜΡ ΘΥ — Meter Theou — Greek for “Mother of God,” i.e. Mary.
So this fresco depicts Mary praying, but what is the story behind it?
It comes from the legendary tale of her “Dormition” — which means “Falling Asleep” — that is, her death. Earliest Christianity left no tradition about what became of Mary. It was not until the 5th century that stories giving varying accounts of her death began to appear.
The tradition in iconography relates that one day Mary went to Mount of Olives in Jerusalem, and prayed that her soul might be taken from her body so that she might see her son again. The angel Gabriel appeared to her and told her that her wish was to be granted: she would die in three days.
So that is what we see in this fresco from Dechani — the “Prayer of the Mother of God on the Mount of Olives.” This example does not show the angel (some do) — just a hand blessing from Heaven.
Now you may notice the stylized trees are all bent in Mary’s direction. That is because tradition says that when Mary said her prayer on the Mount of Olives, the trees bowed to her in reverence (we also find the motif of “bowing trees” in icons of Irene Chrysovolantou).
It is not surprising that we seldom see this subject on its own. Usually it is depicted as one of the scenes in detailed icons of the Dormition, such as this 17th century Russian example from Yaroslavl:
The image of the “Prayer of the Mother of God on the Mount of Olives” (Моление Богоматери на Елеонской горе/Molenie Bogomateri na Eleonskoy Gore) is the second scene from the left in the bottom row. Though small in the photo, we can see that it nonetheless has the same basic elements as the Dechani fresco.