This Slayer has experienced a fate far worse than death for any Dwarf that has taken the Slayer oath — being captured!
In the Warhammer fluff, Dwarf Slayers are those who have been shamed in some way — Breaking an Oath, failing to exact retribution for a family or clan grudge when given the opportunity, etc. The shame of this kind of thing causes them such grief that they shave their hair into a stiff mohawk, dye their remaining hair & beards bright orange, and swear blood oath to never rest until they find an honourable death in battle against the mightiest of foes.
Because of this, the enemies of the Dwarfs find great pleasure in capturing these (extremely powerful and borderline psychotic) slayers when they can, and just holding them captive so that they cannot complete their oath. While this doesn’t happen often, when it does it is a great victory for the capturing army (Usually only Orcs, Goblins, Skaven, and Dark Elves find enjoyment in this psychological cruelty — other armies of Chaos & Destruction care more about killing & death to capture anything, and the other armies of Order have too much respect for the Dwarfs and have too many other battles to fight to worry about capturing a Dwarf Slayer).
In-game, this piece would be used as an Objective — Either the Dwarf Army will be attempting to rescue this unfortunate Slayer, or they will have come across him and get ambushed by the capturing army and must defend him.