Krell must always issue a challenge whenever possible, and must answer any challenge issued by the enemy. If Krell is fighting a challenge whilst in the same unit as Heinrich Kemmler, he has the Heroic Killing Blow special rule.
Source: Warhammer Armies Project: Unofficial 9th Edition (v2.2)
Krell must always issue a challenge whenever possible, and must answer any challenge issued by the enemy. If Krell is fighting a challenge whilst in the same unit as Heinrich Kemmler, he has the Heroic Killing Blow special rule.