This is not to be confused with an elf bladder, something different, though not necessarily altogether different, as the elf bladder could be the bladder of a bladder elf.
No, a bladder elf is, in a similar vein to other professional elves, the shoemaker's elves, the wood elves, the Keebler elves, etc, an elf who steals into your bedroom late at night, and fills your bladder, so that when you wake up in the morning, you can have that refreshing mad dash to the bathroom for your morning pee. Less prestigious than the other elven professions, to be sure, but still fraught with dangers and hazards.
This particular bladder elf was unhappy with his lot in life. Sighing wistfully one evening, while filling a particularly large bladder, a passing by bladder elf noticed him, and talked to him of Herr Christopher Hoffe's farm, where there was little work to be done, and there were always large bowls of porridge for the happy bladder elves to eat.
And so, the little bladder elf happily took off for the farm, where life was good for a while, until the Bladder Elf Union, who had gotten wind of his unauthorised assignment change caught up with him and had him disbarred and expelled from the union.
Humiliated and jobless, the bladder elf wandered aimlessly for days, until he found himself in Madagascar where he was promptly eaten alive by a rare and endangered species of carnivorous primate.