Easy to achieve:

in Rune.ini:
FullscreenViewportX=1920
FullscreenViewportY=1080
FullscreenColorBits=32

Have fun

Thanks so much for posting this! It's amazing to be able to play Rune in HD.

Because original Rune is obliviously an OLD game that came out in 2001 and would primarily support 4:3 resolutions from the video options from the outset, not to mention the game's Direct3D renderer relies on DirectX 7 which has compatibility issues with running properly/functioning on modern versions of Windows past 7 (especially on Windows 8/8.1). Rune Classic did try to fix this and feature widescreen resolutions within the video options natively for widescreen displays, but sadly Human Head Studios still decided to use the outdated Direct3D renderer in the updated version rather than fixing the renderer with one that supports DirectX APIs like DirectX 9 or 11 that are still relevant, and to make matters worse, Rune Classic cannot use custom fan-made renders like the Direct3D 10 by Kentie or OpenGL ones for whatever stupid reason the developers did to botch that feature. Contrary to the update that came exclusively tot he Steam version of Rune Classic, the game still uses DirectX 7 and NOT DirectX 11, so the devs has been lying to players about Rune Classic having switched over to DirectX 11. It is possible to use dgVoodoo 2 to wrap Rune and Rune Classic's outdated Direct3D renderer to DirectX 11/12, although you might need to use Windows 7 compatibility mode on the game's executable to fix the widescreen options not showing in Rune Classic.