A minor correction -- death is not actually "inevitable." The tetrodotoxin molecule does not block the Sodium channel indefinitely. If you can stay alive for 24 hours (through luck or a forced-breathing machine), the molecule will un-bind and you will be fine.
That said, it's heinous stuff, 10,000 times more lethal than cyanide, per gram. 25 minutes after exposure it begins to paralyze its victims, leaving the brain fully aware of what's happening. Death usually results within a few hours from suffocation or heart failure. TTX is one of few neurotoxins that are not protein-based. It is found in a wide array of animals that are related only very distantly. The explanation may be that it is produced by a microorganism that lives in symbiosis with each of these creatures.