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Berek Thunderfist was a previous Wolf Lord in the Space Wolves Space Marine Chapter. Berek was the predecessor Wolf Lord to Ragnar Blackmane. When Berek was slain in mortal combat with a Tzeentchian Chaos Champion, Ragnar led a force that tracked down and slew his master's killers. Afterwards Ragnar was hailed as Berek's successor.¹

It is just a coincidence that Berek Thunderfist has the same last name as Ragnar Blackmane's original tribal name; the reason Berek is named 'Thunderfist' is because he has a bionic arm that sports a built-in Power Fist. The loss of the arm is claimed to be the work of Kharn the Betrayer.³

Sources

• ¹:Codex: Space Wolves (2nd Edition)
• ²:Space Wolf (Novel Series) by William King
• ³:Grey Hunter by William King