Death.
Life is imagined to be a beautiful blessing, however, I believe that is just a mask disguising our true fate. I like to think there is something after death, but it's difficult. Maybe it's a lack of faith or an over-exposure to atheist beliefs, maybe it's just my skepticism in all things unknown seeping through, but I find it hard to believe that any real afterlife exists in any way other than figments of our imagination personified in the tragedy that is death. However, assuming something does happen when we die, I'd like to believe it was reincarnation. Some way for us to continue living forever in countless forms. Maybe reicarnation is almost scarier than a lack of afterlife, immortal beings living and not having the capability of remembering each lesson learn, doomed to repeat the same fate over and over again. The goal of Hinduism is "salvation from samsara" or the end of reicarnation. The word 'salvation' indicates freedom, maybe reicarnation is something to want escaping from. Maybe there is no happy ending possible.
Whether there is or isn't an afterlife, isn't it terrifying either way? It's an endless circle, we say 'forever' right now and that means 'until death', but in actuality, it means for the rest of time, which is endless. The cycle continues and repeats and has no start nor end, so forever is an insane amount of time. The idea is easy to get lost in, when you push past that initially shock of recognising that death exists and no one knows what happens when it comes, you realize that you are trapped in the loop of time and there is never going to be an escape from that. It's horrifying.
Life, as a whole, is terrifying, death is awaiting us through every choice we make. We cannot escape it. It will never be able to end. I understand a fear of death, I hold one, but that fear isn't necessarily justified because we don't have the secrets of death. A fear of pain is more so justified, it's understandable to not want death because of the pain in may hold. I feel like both those fears are quite different, one is a fear of the unknown (death) and the other is a fear of the pain that comes along with it.
There is so much left to be discovered and most of it we will never know.