1.  Forgotton one
2. Revenge
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You know, I used to know the word for revenge and just forgot.  As for
"forgotten one", I assume by "one" you refer to a person.  If so, my
inclination would be "wasurekata"  Given that "wasuremono" means,
roughly, "something you forgot", replace the object "mono" with "kata" for
person, and you get "wasurekata".  I could be wrong, though...

tom