Here's the breakdown:
* The unicorn is a crucial plot point. Harry's encounter with a unicorn in the Forbidden Forest is a pivotal moment in the book. He witnesses Voldemort trying to drain its blood for magical power. This is what leads Harry to discover the truth about Voldemort's return.
* The movie adaptation altered the scene. Instead of showing the unicorn, the film depicts Harry finding a unicorn dead and drained of blood. This allows the audience to grasp the severity of the situation without the graphic imagery of Voldemort harming the creature.
* The unicorn's blood is still important. Although not shown, the unicorn's blood plays a key role in the film, as it's what Voldemort uses to restore his physical form.
Essentially, the unicorn is a symbolic presence in the movie, even though it doesn't physically appear. Its absence highlights the danger and darkness Voldemort brings to the world of Harry Potter.