The seers were an enigmatic, magical race. During the last rise of the Elder Dragons, they forged the original Bloodstone to contain all untainted magic, including the jotun's. They allied with the forgotten, dwarves, jotun, and mursaat to resist the Elder Dragons and survived. During this time, the five races worked together until the mursaat betrayed the other races and fled from the world. In the subsequent war between the mursaat and the seers, the seers were able to develop a defense against the mursaat's Spectral Agony magic, but it was created too late in the war and the seers lost. Seer ruins exist still, but those who find them do not know their origins.
Roughly 250 years ago, one of the few surviving seers aided some Ascalonian heroes in fighting the mursaat and, intentionally or not, releasing the titans. During the Krytan Civil War, Zinn was experimenting with a seer, though it's unknown whether it was alive or dead, and seer magic became an instrumental part of the Shining Blade's plan during the Battle for Lion's Arch.