Unless stated otherwise, all information herein is taken from A Memory of Light, Chapter 37.
Einar was an Asha'man involved in the Last Battle. He was positioned on Polov Heights to help defend the Dragons but was killed by Demandred when he opened a gateway while in a full circle and balefired both Einar and Kwamesa.
"I'm trying to stop him, but he's in a circle. A full circle. I've never seen such power before" (About Demandred).