Is there any way to get the flail of ages after you beat the Keep board? I beat the board and the golem room no longer has the piece. Thanks

As far as I know once you clean out the keep, you can't get any of the treasure back. If the keep becomes your stronghold, the chests are now empty. Otherwise, the drawgate is up.

You can either reload from a saved game prior to leaving the keep, or you can cheat by adding the flail using a program like Shadowkeepr, or the CLUAConsole command (I don't know the flail code off hand, but you could look it up very easily).