Here for 3440x1440 21:9
0) Open your hex editor i use xvi32 and locate dark souls 3 exe (backup first before modifying)
1) HEX
Find this HEX string:
80 07 00 00 38 04
Change it to:
70 0D 00 00 A0 05
2) WINDOW mode
Make the game run in windowed mode, either going in game settings or change the GraphicsConfig.xml (C:\Users\YourNAME\AppData\Roaming\DarkSoulsIII) This folder might be hidden.
Set the .xml file to read only after modifying
<?xml version="1.0" encoding="UTF-16" ?>
<config><ScreenMode>WINDOW</ScreenMode><Resolution-WindowScreenWidth>3440</Resolution-WindowScreenWidth>
<Resolution-WindowScreenHeight>1440</Resolution-WindowScreenHeight>
<Resolution-FullScreenWidth>3440</Resolution-FullScreenWidth>
<Resolution-FullScreenHeight>1440</Resolution-FullScreenHeight>
<Auto-detectBestRenderingSettings>ON</Auto-detectBestRenderingSettings>
<QualitySetting>HIGH</QualitySetting>
<TextureQuality>HIGH</TextureQuality>
<Antialiasing>ON</Antialiasing>
<SSAO>HIGH</SSAO>
<DepthOfField>HIGH</DepthOfField>
<MotionBlur>HIGH</MotionBlur>
<ShadowQuality>HIGH</ShadowQuality>
<LightingQuality>HIGH</LightingQuality>
<EffectsQuality>HIGH</EffectsQuality>
<ReflectionQuality>HIGH</ReflectionQuality>
<WaterSurfaceQuality>HIGH</WaterSurfaceQuality>
<ShadeQuality>HIGH</ShadeQuality>
</config>
3) Start game Go options --> select fullscreen (you need to do this thing every time you restart the game)
Also thanks for making this possible