This appears to be a regular hammer of largish size and extra weight. It will be too imbalanced, somehow, to wield properly in combat, unless the character has 18/01 or better Strength and a height of over 6 ft. The hammer then functions as +3 and gains double damage dice an any hit. If the wielder wears any girdle of giant strength and gauntlets of ogre power in addition, he or she may properly wield the weapon if the hammer’s true name is known. When swung or hurled, it gains a +5, double damage dice, all girdle and gauntlets bonuses, and strikes dead any giant* upon which it scores a hit. When hurled and successfully hitting, a great noise as if a clap of thunder broke overhead will resound, stunning all creatures within 3” for 1 round. Throwing range is 1” + ½” per point of Strength bonus for the gauntlets and girdle. The hammer of thunderbolts is very difficult to hurl, so only 1 throw every other round can be made, and, after 5 throws within the space of any 2 turn period, the wielder must rest for 1 turn.
* Depending on your campaign, you might wish to limit the effect to exclude storm giants and include ogres, ogre magi, trolls, ettins, and clay, flesh, and stone golems.