While there's not much in the way of documented planning for the Reaper Demon, its primary mechanic would later be reused for the Nightmare Demons concept that was featured on the channel. As was the case with many of my scrapped ideas, the Reaper was planned to be a short 3 minute video, similar to the Cursed Skull concept. But in mid 2023 I had decided against this idea as it no longer fit with my vision for the channel.
The Reaper Demon was a rare spawning enemy that would place a curse on the Slayer if they were hit. This curse would begin a time limit of 45 seconds and for that duration you took 2x damage from all sources. To remove the curse, you would need to defeat the Reaper. Once the Reaper reaches half health, they become translucent, just like a Spectre demon and harder to track as a result. If there were more than one Reaper, the damage amplification effect would stack meaning if 3 Reapers cursed you, you would take 6x damage from all sources.