Thunder hammers release a terrific blast of energy when they strike an opponent. A thunder hammer counts as a power fist, but any model wounded by it and not killed may not attack until the end of the next Assault phase. Vehicles struck by a thunder hammer suffer a 'crew shaken' result, in addition to any other results they suffer.
Errata — Chapter Approved: Codex: Daemonhunters Q&A (v1.0): The thunder hammer costs the same as a power fist (25 points) intentionally — it's a deliberate points break to encourage taking one for Ordo Malleus characters, as the hammer is the holy weapon of that order.