When the ultimate betrayal comes to surface, 8 friends will become enemies and only 1 will be left standing.
Some questions pop to mind –
1. Who betrays whom?
2. Who’s your protagonist?
3. What was the nature of betrayal? &
4. ..As a result of which, what becomes the protagonist’s GOAL
PS. If you are aiming to write for multiple protagonist’s, the link below might be helpful:
As variable said earlier, this is a tagline for a movie, not a logline.
As a logline it’s incredibly vague. You could describe a crime thriller in vein of Reservoir dogs or a high school comedy where 8 friends are vying for the same guy for example.
Until we get more details, it will be difficult to help you develop the concept or improve on the logline.