Macbeth has a vision of a dagger floating in the air before him, the handle pointing towards his hand and the tip aiming toward Duncan.
Scene 4: Macbeth becomes king
Macbeth comes out with his hands covered in blood and says "the deed is done" Macbeth becomes overwhelmed and Lady Macbeth tries to calm him as she notices he forgot to leave the daggers with the sleeping chamberlains, to frame them for Duncan's murder.
As Mac-duff enters Duncan's chamber because he was asked to see him this morning, Mac-duff then comes running out shouting "THE KING HAS BEEN MURDERED".
Ross walks outside of the castle with an old man and explains how nature has been out of wack lately an how Macbeth is now being presented as king by other lords.