Short answers, pulled from the story.
The Amduat emerged during the New Kingdom of Egypt, a period spanning from approximately 1550 to 1070 BCE. It appeared first in fragments within the tombs of Hatshepsut and Thutmose I at location KV20.
Ra descends into the underworld as the sun sets in the western horizon known as akhet and travels through twelve distinct hours before rising again in the east. He transforms into a ram-headed figure separated from his physical body while seeking reunion with Osiris who now holds the form of the dead god.
The earliest complete version resides in KV34, the tomb of Thutmose III located in the Valley of the Kings. Thutmose III ruled halfway through the Eighteenth Dynasty during the New Kingdom.
The Amduat divides visually into three continuous horizontal registers separated by vertical textual columns. Each vertical register contains information about the title of the hour and the name of the gateway guarded by a deity.
At the end of the New Kingdom the Amduat lost its exclusivity completely and appeared on both coffins and papyri for deceased people of lower classes than royalty or nobility. It remained reserved almost exclusively for pharaohs until the Twenty-first Dynasty.