I keep having flashbacks of this cartoon I watched on VHS as a child.

Not sure if film or TV series but all I remember is...

There's a man, (possily knight) in a castle tower or dungeon with a sword. There's an evil shadown on the wall and starts attacking his shadow, thus hurting him physically, slashing his back etc. In the end the hero realises he must use his shadow to fight back...

What's the name of it?

Sounds like

Faeries (81)

A young hunter wandering through the forest in pusuit of a deer is magicaly transported to the fairy kingdom by fairy magic. The good faeires must enlist his help in a battle between the good faeries and the forces of darkness led by the kings rouge evil magical shadow.
The story was inspired by the artwork of artist Brian Froud.

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OMG thank you soooooo much! Yesss this is it! Well done :)