SIGIL Announcement
Download:
https://www.romerogames.ie/si6il
SIGIL is 100% free, but you can buy the box version or other merch.
Of course, you need DOOM as well. Essentially .wad file (from the original or The Ultimate DOOM) + GZDoom or some other port.
Yes, SIGIL is for DOOM 1 from 1993, not DOOM 2.

SIGIL (DOOM) Review (Another John Romero masterpiece?)
So I Tried... SIGIL
Doom: Sigil with Heather