This is a mod for Chex Quest (And Essentially, Doom.) I played this on GZDoom. It's a really cool My Little Pony mod where you play as Rarity attempting to rescue your friends, who have been captured by The Changelings. There are many levels to explore and enemies to face. It's pretty fun. I'll be playing it on Nightmare Mode, which is the same as commercial Doom Nightmare. It's a challenge for sure, this second episode is harder than the first, but manageable if you know what you're doing.
You could call it a mod within a mod, as I'm playing the Custom Edition. Same thing, but with extra pretty details. I'll still call it RariDoom as it is still that.
The Blog containing all the information you need, including downloads. http://raridoom.blogspot.com/
The cool person who made the original thing.
/ bahatumay
The other cool person who updated the source material with all the flashy cool stuff.
/ gunman511