There will be those times when we will have someone before us who is eager to become a Christian. He gets it. He understands that he has some serious problems because of his choices. He also understands that Jesus offers to rescue him from all of that. And he understands faith and its flip side, repentance. He is ready to tell Jesus all about this so that he can become a disciple.
However, there will be those times when you will have spent lots of time with someone explaining the ins and outs of the Gospel, answering questions and posing a few of your own. But after all of this, it becomes clear that that person is just not willing to bend his will to submit to Jesus as Lord so that he can enjoy Him as Savior. It needs to be said that there will be some times in these kinds of situations that you will need to say one more thing. You will need to tell the person about hell. Not always, but sometimes. And you don't do this as a scare tactic. But the fact of the matter is that the person with whom you have been discussing the Gospel, this person who has rejected it, this person has chosen to end up in hell. He may not see it in those terms. And that is why you need to tell him.
For whatever reason people think about hell in terms that everyone knows cannot be true. Some guy in red tights and a tail? Pitchforks? The imagery that they focus on gets in the way. So, ignore the imagery and get to the heart of it all. Hell is the polar opposite of heaven. Think of all the happiness and joy of heaven. None of that in hell. Heaven is about enjoying the presence of God. In hell the only evidence of God's presence will be His anger. And while the citizens of heaven will so thoroughly enjoy each other, those in hell will feel incredibly lonely.
Some may hear this as an attempt to terrify someone into heaven. That never works. This is simply trying to make clear what it is that a person has chosen for himself. Hell forever. If you can't tell him about this with deep sorrow for him, maybe even tears, then don't tell him. That's really the only way to communicate the complete horror of hell.