But what if there was a weapon that was actually made this way and magically enchanted to only transfer the generated force when applied as a tool or weapon. In other words it has a "true mass" of an object with density of star's core and a "fake mass" with mass and gravitational pull of a regular warhammer.
True mass applies to the force(power) equations only on succesfull strikes that land on the intended target. If the wielder misses, drops the hammer or hangs it on a coat rack it behaves according to its' fake mass.
How much force a regular blow would apply? What about Thor's signature move?
If there are negative effects like recoil(Newton's Third Law) consider them being magically nullified(behaves like fake mass) - the goal is to determine how powerfull as a weapon or tool it could be with only practical applications being possible.
Neil posts that a White Dwarf core has a density of 1 tonn per cubic cm, while a Neutron Star of 1 trillion per cubic cm. I'd be content with a White Dwarf but Neutron Star is also appreciated.
Bonus question: if the magic is dispelled what havoc such an object would wreak on a Earth-like world just by lying on the ground.