I'm pretty sure you don't meet the minimum requirements for the game. Dying Light requires a DirectX 11 capable graphics card. The 8400GS is only capable of DirectX 10, and is much slower than the GTX 560 or Radeon HD 6870 which are the minimum graphics cards for the game. The game also requires a quad core CPU, you didn't specify if whether you have a dual core or quad core CPU.
Bottom line is, you need new hardware in order to play. You definitely need a new graphics card, and you probably will need a new CPU/Motherboard and possibly RAM depending on how old your system is. If you want to play modern games, it's probably time to do a completely new build if the rest of your components are as old as your graphics card.