To obtain one, either complete the Enchanted key miniquest, or trade with another player. However, you can only receive one Guthix mjolnir during the Enchanted key miniquest.
The Guthix mjolnir is a two-handed members-only weapon. There are no skill requirements to wield this item.
It is obtained from the new treausure hunts by using the enchanted key to get it. You have to use the enchanted key and find the exact spot where it is and dig on that spot to get a chance of getting this weapon. This weapon is also 2-handed. After you get all three mjolnir's the key vanishes. This weapon is a hammer NOT a spear.
A Guthix Mjolnir.