For a character with over 10K with it and after wished for werewolf king and did a lot of killing I got no decrease in Willpower. It was native spear of surrendering so question is wide open. A Dark Elf Paladin of Level 1 found a Rapier of Surrendering in the SMC, used it a handful of times before switching, and incurred a -1 to Willpower after selling it at Level 4.