The Tainted are Fairfins who Dreamer became Nightmares and their link was cut before they became fully corrupted.
It is also possible for a Fairfin to get corrupted but a creature that can corrupt others but it's much harder to corrupt a Fairfin then other creatures.

The link between the Fairfin and Dreamer could have been cut different ways, the Dreamer could have cut as their last will before turning into a Nightmare, the Fairfin could have cut it themselves, the Dreamer could have been killed in the process of fully turning into a Nightmare, somehow they got separated enough to cut the link, there many possibilities.
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It is possible to save a Fairfin from becoming a Forgotten, it their link is cut before the corruption fully overtakes them they can be saved, but they have to be returned to the land of dreams, and taking to a special place called ' Suru Namida' the birth place of the Fairfins...I'll tell you it's located in the 'The lake of Hope' their a ritual bath that can be performed that will remove the darkness in them. But without their Bond partner their life is never the same, the survivors are referred to as 'the tainted' and will be darker in color then normal Fairfins, these Fairfins can still make Bond with someone else but rarely do, because they have already failed ones and fear to repeat it, and no one could ever replace their first bond partner.

Though they are very rare and out of the Fairfin varieties including the Forgotten, the Tainted have the lowest numbers.
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these Fairfins are put in special pools just for the Tainted, and have assign caretakers to care and look out for them.
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The Tainted Fairfin are darker in color and Light, they sometimes have a backlight. They are also the only Fairfins who flare the color black for a display of their motions.
Their pupils are slit and have darker sclera.

these Fairfins are often moody, and have major mood swings they are ether depressed and mopey, some are angry and bitter, and some just go cuckoo, some are too others, it all really depends on the Fairfin and how they take losing their Dreamer or how their link was cut.
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It is possible for a tainted Fairfin to form a new bond with a Dreamer, but it can be pretty rough for both the Dreamer and Fairfin, the Fairfin is under a lot of emotional stress and would take a special Dreamer to take one in.

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The Dreamers that usually form a bond with a tainted Fairfin are ones who first Fairfin have died, either an accident or they were killed.
It isn't uncommon for Nightmares to hunt and eat Fairfins, Dreamwaker like to eat a Dreamer Fairfin in front of them and leave the Dreamer to start feeling the choke and without a Fairfin a Dreamer can not teleport so they are left stranded.
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It is very, very rare that a tainted Fairfin will form a bond with a Dreamer fawn.

Their abilities are the same as other Fairfins.

( that all, their really not much to talk about them )