John Romero has finally added another expansion to Doom and its totall free:
https://www.romerogames.ie/si6il


You can run it with GZDoom in Linux

Grab a copy of GZDoom and place the SIGIL files into "/home/username/.config/gzdoom" so it can easily find it. You can then use your favourite Doom launcher or run it simply with a quick terminal command like so:

gzdoom -iwad DOOM.WAD -file SIGIL.wad