The eldest of the Seven Fathers of the Dwarves, the first of that race to be created by the ValaAulë. He was set to sleep under the mountains of Middle-earth until after the awakening of the Elves.
When he woke, he journeyed in the wilds until he came upon the Mirrormere, the lake that the Dwarves call Kheled-zâram. Looking upon mysterious stars reflected in its surface, he took this as a sign, and there founded a great city tunnelled into the heart of the Misty Mountains. This was Khazad-dûm, later called Moria.
Durin lived to a great age even by the measure of the Dwarves, but though he was called the Deathless, he was not immortal. He died some time before the end of the First Age, but he founded the line of the Kings of Durin's Folk that extended down through the history of Middle-earth.
Notes
1
According to the essay Of Dwarves and Men in volume XII of The History of Middle-earth, the name Durin '...appears to have been simply a word for 'king' in the language of the Men of the North of the Second Age'.
The name's actual literary source is Durinn in the Old Norse Dvergatal. The meaning there is far from clear, but it can be interpreted as 'one who sleeps'. That would make sense, given that the Dwarves were set to sleep under the earth until after the first Elves appeared, and Durin was said to be the first of the Fathers of the Dwarves to awaken at that time. Other sources prefer the interpretation 'doorward', and this is also a realistic possibility. Though 'doorward' does not have any obvious relevance to Durin I, it does have a logical connection to Durin III, who made the famous Doors of Durin to guard the West-gate of Moria.