Dumac (also known as Dumac Dwarf-Orc  , and  Dumac Dwarfking ) was a  Dwemer king, head of the clan in Morrowind (or Resdayn, as it was known back then). He is known to have been in attendance of the coronation of King Gorieus in 1E 461.
Indoril Nerevar, they formed the First Council, an alliance of Chimer and Dwemer, against the Nordic threat. They were successful in stopping the Nordic Invasion.  
First Council, it is noted that the Rourken Clan refused to join him. On the  Chimer side, House Dagoth, the Sixth House, was absent.
Unbeknownst to him, one of his chief council members,
Kagrenac, was working in secret to create a new Dwemer God. The Chimer found this out, and the First Council broke down, and the War of the First Council began. During the resulting  Battle of Red Mountain, Nerevar killed Dumac. 
Nerevar at Red Mountain ↑
The 36 Lessons of Vivec, Book 36
Five Songs of King Wulfharth
Rislav the Righteous
Pocket Guide to the Empire, Third Edition: Morrowind ↑
War of the First Council
Dwemer Inquiries Vol I
The Seven Curses ↑
Varieties of Faith in the Empire