John Carpenter's Tales of Science Fiction: The Envoy (John Carpenter's Tales of Science Fiction: The Envoy #3)

Good News and Bad News: We've made successful alien contact and, as a result, cured cancer. But is that the good news, or the bad? Because there's a downside. Because there is an opposite number to the Envoy, a gracile golden visitor who has come to heal: the flipside visitor which looks pretty much like Lovecraft's worst nightmare.