You'll notice in the Gospels that demons always speak the truth. They never lie. But they are different entities from Satan, who is a master of deceit. Nevertheless, Satan can also speak the truth (Matthew 4.6, for instance). Sometimes the truth mixed with half-truths and lies can be quite deceptive.

I'm trying to outline my book, which is metaphorically describe the spiritual life of Christians, my idea is that the antagonists (demons) in one scene confess to the protagonists what evil they plan to do to their parents who did not follow them (protanonists). The reason why they tell them this truth is to try to dissuade them from going on their path (Heaven) and to make them feel fake guilty. That's why I ask, can demons speak the truth or are they by nature incapable to tell a truth (John 8:44)?