Herger

 was a Viking chieftain who encountered a Talessh and defeated it, placing it's shield in a mound in Arizona. Constance Clarke later read his warning in the mound. He was stuck in the transmat buffer in the shield and was released about 1000 years later and destroyed the shield when the Sixth Doctor was trying to stop a rogue Talessh. (AUDIO: Shield of the Jötunn)