Every time you see the Base menu (including exiting from another menu such as conversations), there is a 1% chance you will see the message "You feel an evil presence watching you..."
This unlocks the One-Winged Angel quest.

Welcome to the Sephiroth Challenge. I have added a Sephiroth boss fight to SRPGS Acid Trip 2.5, it's unlocked through the method above and it may just very well be impossible. In order to incentivize people to do this challenge, I have decided that the first ever person to clear this challenge will be granted a very special reward: They will get the "150$ Tier" tier and all its benefits on my Patreon for free, that's a 150$ value!

Sephiroth has insanely high stats that only get higher the longer you play the game, he's immune to critical hits, has all 3 Sword Skills, and many more surprises. Do YOU have what it takes to beat Sephiroth?