The Rule of Giants (Jötunnlagen) is a period after the Rule of Dragons and the War of Ages when the true giants defeated the dragons, nearly wiping them to extinction. As most surviving dragons fled to Aelantir, the giant realms divided Halann amongst themselves. The start of the Rule of Giants (and thus the end of the Rule of Dragons) is widely debated, and scholars have generally thrown around the estimate of 100,000 BA.
The end of the Rule of Giants, however, has yielded more conrete data, and date the end as 12,000 BA which marked the end of Halann's last ice age, and also the end of the last of the true giant states: the Realm of the Ice Giants. It is said that the last of the dragons who remained in Halann in defiance banded together to melt the icy realm of the Frost King Gerud, in hopes of drowning even the tallest true giants. It seems that the dragons did succeed, with the true giants declining massively since 12,000 BA. It is said that the Giant's Grave Sea is named after this act.