A translation of Amon Amarth in the elven Sindarin tongue, Mount Doom is the name given by the men of Gondor to the Volcano Orodruin, which although less than 5000 feet high stood alone and towered over the plains of Gorgoroth in the North of Mordor. The sides of Mount Doom were a mass of ash and burnt stone. Entrances in the side of the volcano led into its heart, known as the Sammath Naur (Chambers of Fire) in which were the huge fissures called the Cracks of Doom. The fires here were invaluable for Sauron in the forging of weapons and sorceries, and here was the One Ring forged, as detailed in The Silmarillion. The Cracks of Doom produced the only fire in Middle Earth hot enough to bring about the destruction of the Ring of Power.
Frodo and Sam struggle across Mordor to Mount Doom, and scale its sides to come to the Cracks of Doom. The painting shows the moment at the Cracks of Doom when, in the red light of fires burning far below, Frodo finally succumbs to the power of the ring, decides he cannot destroy it and will take it for himself. He places the ring on his finger, and the Eye of Sauron sees him, focussing his whole will and purpose on the mountain. His hordes, battling with the armies led by Aragorn and Gandalf at the gates of Mordor, are forgotten and left leaderless and confused. The Ringwraiths turn and hurtle towards the mountain. At this moment Gollum makes his final appearance. Knocking Sam aside he flings himself at Frodo and bites off his ring finger, before stumbling into the cracks of doom and causing the ultimate destruction of the Ring of Power.
With the destruction of the Ring came the destruction of all of its works and the fall of Mordor. Orodruin flared, the mountain shook and the tower of Barad-Dûr and the black gate of Mordor fell.