Psychometry!
The practical result looked a lot like a New Californian or Hersheyan kinetoscope production, only in reverse. As the spell was cast, dim outlines of bones could suddenly be seen under the ground. As the spell steadily went to older and older memories, those outlines shifted in place slightly — and then seemed to erupt out of the ground, only to flow to various spots on the battlefield. There they lay, until they began to stand up, and make various jerking motions, miming the use of swords, shields, bows, and various pole-arms.
It obviously wasn’t a perfect representation, since we couldn’t see the skeletal outlines of everybody who survived the battle. But it was easily enough to let us get a feel for how the fight had gone — and, more importantly, it let the researchers steadily peel away and identify the layers on impressions that had to do with the original fight. Anything left would be what we were looking for, in terms of wandering ghosts. As a technique, it was really very impressive. Hank had put together a good team.
And then it all went to hell.