Nightpool was a medicine cat of ShadowClan. He had black fur with a silvery underbelly, tail-tip, and back; he had deep crystal-blue eyes as well. He was born in ShadowClan under unnamed parents and became a medicine cat apprentice due to his strange dreams and visions, along with a knack for herbs. He became a very experienced and skilled healer, and took on two apprentices in his lifetime: Sparrowflight and Honeyleaf.
While he was still a young medicine cat, ShadowClan had temporarily banned interacting with other Clans except at Gatherings and at times of great need due to a traitor spying for RiverClan, who had masqueraded as a ShadowClan warrior. Despite this, Nightpool slipped out of camp in the darkness and went to WindClan to help a sick kit, whom he had learned of at the Gathering two nights before. The leader, Arrowstar, discovered this, and exiled Nightpool from the Clan. Nightpool then took to the woods beyond the territories of the original four Clans on the lake and found TunnelClan. An unknown warrior invited him to camp as a guest, and Nightpool was allowed to live at the camp for a few weeks before returning to ShadowClan territory to visit a friend. Suprisingly, he was admitted inside the camp and reinstated as medicine cat, and took on Sparrowpaw as his apprentice. Sparrowpaw then became a full medicine cat and was known as Sparrowflight (not to be confused with the TunnelClan Sparrowflight). Due to Sparrowflight's being confined to a different part of camp for a long time because of a strange fatigue in her legs, Nightpool took Honeypaw, a young golden she-cat, as a second apprentice. Even though she became known as Honeyleaf when she was made a full medicine cat, she stayed on as his apprentice, as did Sparrowflight when she returned to her duties.
Nightpool had a secret affection for Honeyleaf, though learning to keep it hidden. When foxes were detected in the vicinity of the ShadowClan territory, Honeyleaf and Nightpool's sister Darkflower, who was an elder by now but liked to help Nightpool and Honeyleaf, went to collect herbs. They were guarded from possible fox attacks by two warriors, Mapleleaf and Moorlily. Unfortunately, the foxes did attack, and all four cats - including Honeyleaf - were killed. When he heard the news, Nightpool was so distraught by his grief for Honeyleaf and Darkflower (particularly Honeyleaf), he ventured out into the forest alone to collect herbs in a certain area of the territory, which he went out on to collect herbs and take walks to calm his nerves. While examining some deathberries (otherwise known as yew berries), a fox came across him; this fox had been tracing the scents of Honeyleaf, Moorlily, Darkflower, and Mapleleaf to the camp. The fox attacked Nightpool and was leaning over him to bite his neck once more, when Nightpool shoved the deathberries down the fox's throat with the last of his strength. The fox died, as did Nightpool. His body was discovered by a patrol, which had been tracking the foxes, soon afterwards.
Though he died and was buried in ShadowClan, Nightpool often delivers dreams and prophecies to select TunnelClan cats. He does this and continues to do so because he remembers TunnelClan, who helped him in his time of need.