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