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Umbras are invisible specters, even to witchers, and can only be seen through the use of powerful hallucinogenics (like graytop) or by certain animals, like horses. They can take on any form as a specter.
An umbra is merely an unclean conscience, one tormented by guilt and unforgiven wrongs. A man wallows in sin with loose women and can't sleep at night, then invents hokey about some umbra or phantom and blabbers about it at the tavern.
– Sabine of Dal, mystic and priestess of the Prophet Lebioda.
According to some philosophers, everything we do in life leaves a permanent trace in the delicate matter which makes up our souls. Evil deeds, especially harm done to innocent creatures, can torment the soul of a sinner an cause it to remain restless, even after death.
While traveling through Toussaint, Geralt discovered there might a be grain of truth in these theories and folk beliefs. He came across a hermit whose sleep was troubled by some invisible phantom.
Geralt determined this phantom was an umbra, the ghost of a recently-deceased knight who had committed a terrible deed while alive and now visited the hermit to demand forgiveness. In his wraithly form he drank her blood and systematically deprived her of life energy.
The umbra appeared to the hermit as a horse, for the evil deed the knight had committed was to flog his own mount to death. The knight's spirit was unaware of the demonic role he had assumed in the afterlife.
If Geralt/Roach refuse to forgive the knight:
Such an umbra could be defeated with a silver blade, preferably one coated in wraith oil.
If Geralt/Roach forgive the knight:
Geralt managed to figure out the key to freeing the knight's ghost was to forgive him in the name of the hermit.