Like watching a dying Galápagos tortoise

This one was sitting for quite a while and today I finally found some extra time to spend on it. I started with a good old hand sanding treatment with 400 grit, but pretty soon I moved to the bench grinder to fasten things up. After I was quite pleased with the results I returned to the hand sanding...until...I....found....THIS! :dropjaw:
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I wish I wouldn't ever seen it and everything was much better right now...I don't know if it was there all the time and I just "revealed" it, or accidently made it myself by heating the blade too much with the power tool and by that causing the metal to fail.
I stopped everything and start to wonder if it's still worth the effort, though I think I know the inevitable answer I thought maybe, just maybe there's still a glimmer of hope for this one, what do you think?
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Is that a crack that runs a long the spine and into the blade? I don't think I have ever seen that before. If it is I don't think you are saving that one.

Not quite sure what you are referring to here - have you heat-damaged the blade (discoloured, lost the temper) - if so, it will never take a decent edge.

Or are you referring to what looks like either a crack or a cold-shut. If it's a crack it will go right through the metal - bad. If it's simply a cold-shut its only a crease or wrinkle formed during forging the blade, where the heat at the crease edges is below hammer-welding heat. Usually only visible on one side of the blade - never seen one going right through the steel. They aren't usually a problem, they just look bad.

Regards,
Neil

I have no idea what is this weird looking mark! The first thing that I was thinking is that it's a crack in the metal, but the location and the shape of it looks weird, and the fact that it runs over the spine and stops at the right beginning of the front size making it even weirder to me. I'm adding more close up pictures of it, maybe it will help to determine.
http://dor.andromedia.co.il/or/IMG_1758.jpg
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http://dor.andromedia.co.il/or/IMG_1760.jpg
http://dor.andromedia.co.il/or/IMG_1761.jpg
http://dor.andromedia.co.il/or/IMG_1762.jpg

I don't think it was heat-damaged, I just mentioned the fact that I did use a bench grinder and the razor got a little hot at some points so I had to stop and let it cool a little bit before I continue working on it. I don't know if it can cause the metal to crack because of this amount of heat that was created, it didn't get piping hot, just too hot for me to hold it at the moment. But maybe it was already there from the begining and I just couldn't see it because of the patina and the age marks.

That is weird looking. I would bet on what Neil called a cold-shut. I don't think I would put too much more power to it though. Keep going...what else can you do ? It may shave well after all.
Everybody has a concept in their mind of what metal is. Once you start working with it that concept changes a little.