To access the filter, open your Instagram Stories first. In selfie-view, open the new filter, which will first show you in pretty mascara and lipstick.
As you can see above, when you open your mouth you turn into a scary vampire with blood dripping down.
Flip the camera and you can try it on your friends too.
Along with the filter, Instagram also added some Halloween emoji shortcuts. When you go to comment on a post, these three emojis (🎃 👻 💀) will appear with frequently used emojis.