Some thoughts on the fate of the sons of Fëanor
The consensus amongst fans seem to be that after death, the sons of Fëanor would be kept in Mandos for some period of time before being embodied / released, despite swearing "everlasting Darkness" upon themselves if they fail to uphold their Oath.
For a long time, I've preferred them to actually be in some form of Darkness, whether that be in the form of utter annilation of spirit or being in the Timeless Void. I have been trying to understand why I hold that opinion, and I think it's because I really like the sons of Fëanor, and was trying to make their fates (and hence their characters) more dramatic, more tragic, or simply more special. Existance, albeit an unhappy one, outside the norm of the universe, as eivdence for the amplitude of their misdeeds and suffering.
But in the end, they were really not special at all, were they? The Oaths compels them, and they resist it when the war effort is going decently and their enemy is strong, and then succumbs to it when they think they could take advantage of the weak. Their worthiest deeds are accomplished during (relatively) fair weather, their worst deeds occur when they were already at their lowest. It is a very mundane existence, never really rising above or sinking below what could be reasonably expected of the circumstances. Thematically, they don't deserve an extraordinary fate, even a negative one.
Maybe it is a strange way to think about things, but to consider anyone, even oneself, beneath redemption is in and of itself an act of despair and arrogance. I now like the 'temporary stay in Mandos' interpretation a lot more, and feel like I learnt some kind of lesson in humility along the way.