A Grimwrath Berzerker is what happens when a lodge finds a warrior whose flesh can carry more of Grimnir than flesh should hold. The Zharrgrim hammer rune after rune into such duardin until they walk haloed in furnace-light, then loose them ahead of the fyrd, where a god's fury can do the most arithmetic.
Every Grimwrath swears a great oath before each war — a tyrant to behead, a monster to gut, a gate to hold — and death itself is expected to wait until the oath is kept. Lodge-histories brim with Grimwraths who took mortal wounds at dawn and finished their promised slaughter by dusk before consenting to fall. They fight alone not from pride but from courtesy: no shield-line could keep pace, and none should have to try.