A poem, with an introduction providing cultural context
Every sentient species is wiped out by its own evolutionary heritage. War, climate change, and resource depletion are the most common ways this happens, obviously. There are some interesting outliers, though, as in the case of a species called the Bolants. They avoided most of the common traps, being sociable enough to cooperate, clever enough to be careful, and individualistic enough not to all charge off the same cliff. Their doom was a single recessive gene and an unfortunate cultural position.
The gene controlled a neurological disorder that caused suicidal ideation of increasing severity, beginning several years into Bolant sexual maturity. Sufferers of this disorder inevitably killed themselves, usually before what would otherwise have been the middle of their average lifespan. Unfortunately for the Bolants, this inevitable self-destruction struck a very deep chord in their culture, being seen as tragically romantic in the extreme. Once a Bolant felt the first stirrings of suicidal thoughts, they immediately became the center of attention for those around them, seen as a living tragedy of incomparable fascination. Love affairs with suicidal individuals came pre-equipped with their own dramatic endings, and the logic of "All I have left of him/her is our children" was seen as unanswerably persuasive.
Thus, the Bolants accidentally maximized the reproductive fitness of people with a terminal illness. By the time the nature of the problem became clear, the once-recessive gene was present in a large majority of the species. Worse still, the general cultural response was tragic acceptance, since that had long since become lionized as the best and most admirable emotional state. Now their entire world was doomed to kill itself, a most inspiringly sad state of affairs.
The last surviving Bolant threw himself off a cliff in a state of beautiful melancholy so deeply bred, and so refined by centuries of hard work, that the human mind could not begin to understand its nuances. He had chosen his cliff well, and was quite thoroughly smashed to jelly at the bottom.
This is an undistinguished but typical example of a Bolantic poetry form called a symmetrodon, in which each line has a number of syllables determined by a Fibonacci sequence, first expanding and then contracting. Each line with a given number of syllables rhymes with its counterpart. The central, and thus longest, line must have an even number of syllables and must be wholly alliterative in two even segments. In this example, seventeen syllables are taken by words starting with the letter M, and seventeen by the letter S.
Bright fast years
Laughing, loving, long
Then, one day, that inner twitching
That says I’m chosen by the angel of suicide
That nothing in this world can make me fear death except for my own self-destroying hand
My mirror-masked murderer mocks me mercilessly, making moments seem slow, still slipping swiftly, soundlessly, sinking suicidally
And with the passing of joy and all hope, nothing left but to choose where I’ll make my stand
Inevitability makes me feel dead inside
Unmoved by what was bewitching
One last metric song