Kusunda is one of the most enigmatic languages on the planet. This tongue from western Nepal looks nothing like any other language, and its strange structure baffles linguists. And there's only one person left in Nepal who speaks it.
Kusunda is an example of a language isolate, meaning a language with no known relative. Of course, this probably doesn't mean the language developed in isolation - in practice, it means that any connections it might have with other languages are so ancient that its relatives have all either gone extinct or changed so much that any links are no longer detectable.
There are dozens, perhaps hundreds of such language isolates still spoken today, although most are on the verge of extinction. The language most famous for being an isolate is probably the Basque language spoken on the northern border of Spain and France. That said, the 650,000 Basque speakers are utterly dwarfed by the 78 million speakers of the world's most vibrant language isolate: Korean.
As such, language isolate status is definitely not necessarily a death sentence, but things are looking very grim indeed for Kusunda. The native tongue of a tribe of hunter-gathers in western Nepal, Kusunda was barely known until the 1960s and it wasn't until 2004 that linguists met Gyani Maya Sen, who is now the last fluent speaker of the language living in Nepal. It's thought that there's about six other fluent speakers of the language, but they have either left the country or their whereabouts are unknown.
BBC News has a great overview of the language's current sad situation and the steps that would need to be taken to save at least the knowledge of Kusunda, if not the language itself. Whether it's truly worth devoting extensive resources to saving dying languages is admittedly a tricky question that really deserves its own discussion. (I'd personally say it absolutely is worth it, but I can certainly see the counterarguments.) Either way, Kusunda is a language quite literally unlike any other on the planet, and its loss would represent a tragic reduction in the seemingly endless varieties of human expression and communication.