Ether 15

 Obviously, Ether was a near and clear witness as he wrote - 

"...when Coriantumr had leaned upon his sword, that he rested a little, he smote off the head of Shiz.

31 And it came to pass that after he had smitten off the head of Shiz, that Shiz raised up on his hands and fell; and after that he had struggled for breath, he died."  (Ether 15:30–31)

We do not know if Ether had any contact with Coriantumr after that.