"Unmercey" doesn't appear to be a standard English word. It may be:

1. A misspelling — possibly of "mercy" with a negative prefix, though "unmercy" (without the extra 'e') isn't standard either. The usual word is "mercilessness" or "ruthlessness."

   

2. A coined/creative term — sometimes seen in fantasy writing, gaming handles, or brand names.

3. A typo — perhaps for "unmercy," "unmerge," or a username.

Could you give me more context (where you saw it, the sentence it appeared in)? That would help me pin down what's meant.