You can use mortal to show that something is very serious or may cause death.
致命的 • The police were defending themselves and others against mortal danger. 警察在保卫他们自己和其他人免受致命的危险。
You can describe someone as a mortal when you want to say that they are an ordinary person.
凡人 • Tickets seem unobtainable to the ordinary mortal. 票对于普通大众来说似乎是得不到的。
All human beings are mortal. 人终有一死。