Imbued Northman Kite Shield & Willowisps
 

Willowisps appear to be immune to Slam when equipped with mentioned shield, which is a magic item.

Slam should use shoulder slot; bash uses your shield.

Good to know, thanks! Animation for slam looks like a shield bash, so kinda confusing there.

Makes me then wonder why the Imbued Northman shield is magic, but the armor isn't. But I doubt that is a bug.