"Death is the final unknown." This word occurs after gameover in a PC game. Is it a famous word? Does it have a figurative meaning?
Indeed- if those that believe in an afterlife are wrong, they won't find out when they die. I have always liked the idea of a five-second afterlife with a big sign telling them that they were wrong before the lights go off.
It's not an established idiom. Any native speaker would understand that Tdol was referring to the final moment of death.
There's the added problem that, if the people who believe in an afterlife turn out to be right, then there's every chance that death won't turn out to be the "final unknown". In the afterlife, there could lots of unknowns!.
I can't even find that sentence in the link but it doesn't matter - yes, that sentence is completely ungrammatical. It was clearly written by a non-native speaker (as were most of the responses I did read on that page).
It's an incomplete sentence so I don't know what the writer was really trying to convey but I assume they meant something like "They can use many nice words but there are none that can describe ..."