I thought it was an old style grenade, kinda scared me.

A stielhandgranate? It does look a bit like one. Actually some of the grenades back then in WWI had their charges taken out by troops and had hollow cast iron heads mounted on handles like seen here. Very effective trench clubs. Grenades such the zeitzunder granate and the rohr granate
Thoe grenades were cast iron tubes and balls that had cardboard handles. The german ones were sheet metal charge heads with a wooden stick for the handle.

Needs a cotter pin so the nut doesn't come off. <_<

It's screwed on to the wood. I threaded it on there by hand it made the most god awful noises imaginable.

My only question is where do you find a castle nut that big? My only guess would be that it came off of the suspension of a piece of heavy equipment or a semi truck.

It was given to me by a diesel mechanic and those guys run into a lot of interesting metal objects.

There's always good metal getting thrown out in those shops.

Yeah, good thing he understands how important all of it is to a smith. He sees I can make the best use of it and I return the favour by offering my services to him when ever he needs them.