The hammer. I never played the old Graal PC game so I don't know how the hammer was used as a weapon against other players. But here's how I see it:
Shorter range than sword, does 1 full heart damage, has a one second "cooldown" period (during which time you can't use ANYTHING, much like the net as of a couple days ago, aka a global cooldown). That one second is just an estimate but basically you'd be able to kill someone standing still at the same rate you'd be able to kill them by spamming sword. So its prime use would be for burst damage. But it wouldn't be overly powerful due to the short range and cool down.
And of course there would be a long and awesome (and difficult!) quest involved in obtaining it. And maybe, just maybe, they could sell "customized" versions of it that just change the appearance of it (kinda like the custom bombs, except purely cosmetic).
Another idea instead of it doing 1 full heart damage is for it to strike multiple people at once (that are standing close together), maybe 3 people maximum, but it would only do regular damage and the cooldown still applies (but perhaps it would be shortened in this case).
I think it could work without throwing off balance, but at the same time giving us a fun new item to work with. And obviously it would also be used to get by those posts to gain access to certain areas (much like the rat morph).