Giamo the Tyrant was ruler of Galla when it was an empire at an unknown time prior to the events of the 5th century HE. He used the Dominion Jewel to conquer parts of the Eastern Lands surrounding Galla to build his empire. Nahom Jendrai used Giamo as a reason why it would be devastating if the Jewel fell into the wrong hands.
Emperor of Galla
Giamo was first seen wearing the Dominion Jewel in 75 HE, and thus started his conquests a year later as he took most of eastern Scanra. Then the Eastern Lands were thrown into a series of wars called the Tyrant Wars, or the Gallan Wars, as Giamo conquered parts of Tortall and northern Maren. Eight years into the conflicts, Giamo was assassinated by one of his bodyguards in 90 HE. The throne passed to Parrac the Quiet, who continued the conflicts.
As Giamo is known by his epithet and not by any family, it is unknown if he is a direct or indirect ancestor of the ruling family of Galla in the 5th century. As the Gallan royalty is known for its ancient bloodline, it is likely that he fits in there somewhere.
Giamo is only mentioned in Lioness Rampant and does not appear in any of the books.