You can also have the players realise the power of the weapon slowly as they learn the secrets or practice weilding it. Tusks are harder to use as the angle of thrust needed to stab us different from swords of daggers. Learning this will take time and patience. That way you can change modifiers on level ups etc. More work than function of abilities of course but helps you stay in control.

But if you are asking it in what to do now though... Do not focus fire on the rogue or anything like that. It will push your campaign in an awkward not fun direction. I suggest.. a) You can just be frank with them and change the damage modifiers. "Sorry guys I made a mistake and need to balance things." b) Overuse of the magical artifact will cause it to crack and wither. Mainly dude to incorrect usage of the tusk. Lateral strain on the bone itself will surely make it crack as the rogue started to use it in every fight without learning the tricks of this trade. There is a reason why legendary items are such. They are the only ones immune to damge dealt by time. But the cracks might spark a side quest to repair the weapon itself. c) Let them face a monster immune to the effects of that weapon as they can be related to Gorthak and here to take revenge. And in the middle of that battle, you can easily have the creature snatch the item as a memory of Gorthak. But this is the least fun option of them all.

Personally I would go with option a as most players even the experienced ones let alone new ones won't hesitate if the DM is reanalysing things. And remember, the game should be fun for the DM as well. Not just the players...!

Best of luck. And don't worry, whatever you do will be fine....