Take it to a hydraulic pump repair facility. The shaft you show in the picture looks like it came out of an oil pump or hydraulic pump. I was in the hydraulic repair business for many years and we needed custom pumps built on a regular basis. Often times the shafts required a special cut keyway for a particular application. They don't use a keyway cutter but a grinder wheel since the shaft is already hardened. Otherwise, they need to anneal the shaft, machine the keyway and then heat treat it. However, it will never be as good as the original hardness. Custom machine shops don't usually want to cut on hardened shafts. You may have to hunt around a bit but if you find a shop that assembles custom assemblies of Commercial Shearing hydraulic pumps, chances are good they can do it.
If the shaft is case hardened or is ~50C Rockwell a carbide end mill should cut it. Generally, if you can't mark it with a fine file it's probably too hard to cut with a carbide tool. If it's too hard find a shop that does EDM (electric discharge machining).
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