What is the "mysterious force" that Melchizedek makes reference to and what does it do in The Alchemist by Coelho?

Your
question isn't exactly clear. There are two forces spoken of. For the
first, in the simplest terms, the mysterious force, "there is a force that
wants you to realize your Personal Legend," is the confluence of energy in the universe
that is in harmonic blend when all parts are working in a synchronous unification of motion.
When this confluence of synchronicity occurs, paths lead to appointed ends, dreams lead to
realization and desires lead to joys. In other words, the force in energy in the universe that
is transferred to productivity in life when all energetic fields are working together in unity.
For the other, the mysterious force is that world force that clouds
over the dream, the Personal legend, each sees and knows in their childhood. It is mysterious
because no understanding is had of where it comes from, except societal expectations at large,
why it obliterates personal visions, or what the aim is.