The bone lord is found in the crypt on the bottom right hand side of the Overworld map, hes in a tomb downstairs from the central room. You have to defeat one skeleton to get there.

He wont do anything without an offering, you'll need an Obol made by placing both broken Obol cards side by side so they fit in battle ( these can both be purchased from the trader in the bottom right hand slot, if neither is there buy what is they'll appear )

Once you have placed them side by side in battle you'll get an Obol and you can make an offering to the Bonelord.