Is there a gory horror movie starring Daniel Radcliffe and Emma Watson?

Nope. They have only appeared together in Harry Potter movies.
The most horrific movie Radcliffe has done was Swiss Army Man. Watson’s most horrific, in my opinion, was Beauty And The Beast.

Swiss army man was a flatulant experience.

The only horror movie I remember was this one and Emma wasn’t in it.
https://youtu.be/VnY0fEV30Wk

@ragingloli – I kinda rely on you for that sort of information ;)

^You are learning about loli well.

@Hawaii_Jake
Another part of the dream was some kind of robotic, tentacular alien, that decapitates a grandma, forces its appendages into the torso like some sort of onesie, and wears the decapitated hat as a helmet, to disguise itself amongst humans. Lots of crunching and squelching sounds while it does it.

I suspect the grandma was a politician.