The War of the Worlds

In the future, humanity has left Earth behind and explored the universe. They've found other life forms, but none as intelligent as them. One day, they find a strange planet that contains a mysterious energy source. They investigate and find out that the energy is actually a living creature. The creature is hostile and attacks the humans. They fight back and eventually defeats it, but at a great cost. Many humans are killed and the creature is destroyed. However, the experience has changed the humans. They now know that there are other intelligent life forms out there and that they're not alone.