Employment advice on impact of extramarrital affair of very married business executive?

I am being used as a decoy to hide a business exec "real" affair with another woman at my work. business exec is married with kids and is moving his office mistress to my group while i am being encouraged to leave voluntarily because he is taking responsibilities away from me. If i don't leave, then he is putting me in the "candidates recommended for lay-off" pool. HR supports mgmnt. this guy is an expat from japan...H E L P !!!