There is no way to know now, but gazing into the wild, unfocused eyes of The Herald, you are almost sure that they always lacked a target, even as he fought. Instead he stared passed you, looking down the road he would have traveled for his dark god had he not met you. -- In Game Description
Tome Unlock : Engage combat with "The Herald" (Rank 17 - Champion Zealot; Located in Troll Country at about : 19k -8k by the lake); he will summon a Minion of Flame early in the fight; defeat his minion to unlock this tome.
Note: The Herald NPC will punt. A lot. And far. Backs to wall is a good idea.
This tome unlock contributes to the The Herald pursuit achievement.