Source: British Museum

This entry on Wikipedia explains the circumstances of the Emperor’s death.

The Battle of Magdala was the conclusion of the British Expedition to Abyssinia fought in April 1868 between British and Abyssinian forces at Magdala, 390 miles (630 km) from the Red Sea coast. The British were led by Robert Napier, while the Abyssinians were led by Emperor Tewodros II.

In March 1866 a British envoy had been dispatched to secure the release of a group of missionaries who had first been seized when a letter Tewodros II had sent to Queen Victoria requesting munitions and military experts from the British, delivered by an envoy, Captain Cameron, had gone unanswered. They were released; however Tewodros II changed his mind and sent a force after them and they were returned to the fortress and imprisoned again, along with Captain Cameron.

The British won the battle, and, rather than being subjected to capture, Tewodros committed suicide as the fortress was finally seized. There is a fuller description of the attack on Magdala with illustrations here.

This is what the British Museum says about this drawing.

“Theodorus, Emperor of Abyssinia sketched immediately after the capture of Magdala. 13 April 1868 by R. R. Holmes F.S.A. Archaeologist attached to the Expedition. One of several drawings, formerly in a Guard Book, made on the British expedition to Abyssinia in 1867-68.”

Some images of the battle

The etching shows the fortress ablaze, following the British attack.
The British army encampment near the fortress of Magdala