What would you say the rarity is on a weapon like that. At a minimum is probably very rare due to adamantium right?
Well, the adamantine property for armor is uncommon (and more thematic for the shield than anything) so that's not the issue. The returning property is more of an problem. If you base it on a Dwarven Thrower then it should be very rare. Note however that the hammer is also a +3 weapon and doesn't have the restriction of not targeting the same target (I ruled in my home campaign that you could make multiple attacks with the hammer) so I would probably make the shield rare.
I'd also make the shield an inherited item that grows in power as the character levels as well. So at higher levels it could get pluses.