Power, it can be strong or weak, for good or terrifying. There are dark forces in the world but they walk on two feet. The “devil” is a term for humans that have lost their way; it is a descriptor of behavior, not a person or entity.
When devils appear in dreams, the image means that the dreamer is about to be presented with a test. The test will be a significant challenge to a core belief. Non passage of the test will have long term ramifications for the dreamer. It is not that a secondary chance to reverse the impact will not be available; it is just that the opportunity will not present itself for quite some time and the dreamer will have to live with the consequences until then.
The next few days after the dream, the dreamer must be very careful to walk their talk. The test will directly relate to a firmly held belief, one that the dreamer has verbalized a position on. The issue will have a spiritual component, but then most things do. The test is not going to be something petty but it will be something that will have a great impact.
When the test comes if you are completely true to your beliefs and react instinctively based in them then it will work out fine. If you hesitate and contemplate the direct course of action then the wrong path will be chosen because you will use human instincts to respond instead of spiritual instinct.