The Berserker Staff is an Asgardian artifact and weapon wielded by the Asgardian army.
The Berserker staff was the primary weapon supplied for the Asgardian army. One of these weapons was possessed by Elliot Randolph (though he claimed it possessed him). After leaving the Asgardian army, Randolph chose to stay undercover on Midgard but decided that his weapon should not be wielded again. He broke it into three pieces and hid each piece in a different area to prevent anyone from finding it.
Centuries later, a group of young paganists found the staff. They want to dominate the world by using the staff and impose their beliefs, however Agent Phil Coulson and his team managed to defeat them and recovered the staff.
When the war between HYDRA and S.H.I.E.L.D. began, the sleeper agent Kaminsky tried to used it against Melinda May. But as he did not know how to use it, May quickly managed to defeat him and retrieved the staff. Currently, the Berserker staff is the property of S.H.I.E.L.D.
People who touch the staff feel a sense of anxiety and anger. This effect wears off on Asgardians once they sleep, but for humans, it wears off in a few decades.