South Korean horror film
Into The Mirror
(2003), a film about a detective who investigates a series if horrible murders, all involving mirrors.
New director Alexandre Aja made lots of changes to the original script, keeping the mirror-based elements and some of the death scenes but otherwise creating an entirely new story featuring detective-turned-security-guard Ben (Kiefer Sutherland).
Most Horrific Element:
Ben's sister is killed by her reflection when it reaches up to its mouth and pulls her jaw apart, ripping her face in two.
The film's not great, but it's a nice moment - created using practical effects, no less.