Mortal
Part of speech: adjective
Definition: Susceptible to death by aging, sickness, injury, or wound
Part of speech: noun
Definition: A human; someone susceptible to death.
Example sentence: All men think all men mortal, but themselves.
Merciless
Part of speech: adjective
Definition: showing no mercy; cruel and pitiless
Example sentence: For years Don Imus was just - boy, he was merciless in his criticism of me. Maybe it was justified, but that didn't mean it didn't hurt.