I know you are asking for a free rogueline, but Caves of Qud has to be one of the greatest games of all time. I don't know anything about this free version you are talking about, but the game is so epic, it is worth the sats.
Discussion
Seems to be an old Caves of Qud which is free, but that might be fine by me:)
https://freegogpcgames.com/4525/caves-of-qud/
And here
https://archive.org/details/caves_of_qud_2_0_206_78_73562_gog_linux
Have you heard of Dwarf Fortress?
Yeah, I was wondering if that would be a good place to start as a total beginner to the format
Oh boy, not sure. I tried the free version, and couldn't get into the interface.
Also not free, but if you want a roguelike that is simple to play, hard to master, and is a ton of fun, check out Legend of Dungeon.
Yeah that looks like it is very old, but there should still be Jopa and the first bit up to Golgotha, so you can get a feel for it before purchasing the full version. CoQ just won a Hugo Award, if you hadn't heard about that here's a link: