Boldest and most curious of the elves of the Golden Age were the Aquatic Elves, youngsters of only a few centuries, who took themselves deep into the seas and oceans of Undar, seeking to awaken the creatures that dwelt in the depths.
Yet they had scant success, for underwater magic is different from and more difficult than land magic and the creatures of the sea were slow to learn and swift to forget, and only among the whales was there a lasting effect, which bore fruit in the mighty being of the Leviathan.
Then it happened that the Aquatic Elves chanced upon the drow, for at the bottom of the ocean floor lay the caverns of the Underdark. Now, being young, the Aquatic Elves had not experienced the war with the drow, and they were bewitched by their fair words and generous promises.
The drow proposed a bargain: the Aquatic Elves would surrender their innate magical abilities, and in return the drow would grant their wish to give sentience to the creatures of the sea.
The Aquatic Elves agreed, and they sent a delegation to the drow to complete the ritual. The magic was long and complicated but in the end the transfer was complete and the creatures of the sea awakened- though many were awakened rather to malevolence than friendship, which for ever after the Aquatic ELves had bitter cause to rue.
Yet even beyond the Aquatic Elves were deceieved, for no sooner was the ritual complete than the drow seized the delegates with nets and clubs and sleep spells, and only a handful escaped back to the ocean.
The rest were taken alive by the drow, and horribly tortured to transform them into a new kind of sea-being, the Sahuagin, or Sea Devils. Yet the drow worked the transformation imperfectly, for one in every hundred Sahuagin is born in the likeness of an Aquatic elf, though these malenti are as evil in spirit as their ugly brethren. The drow thought by this means to conquer the oceans, but the sprit of free will moved the Sea Devils as all drow creations after the Gods, and the Sea Devils hated and still hate the drow with a passion that befits what the drow did to their progenitors.
Thus evil will did evil mar, but still did evil, for the Sahuagin see in the Aquatic elves the faces of their enemy, and persecute them too. Indeed such is their hatred for the elven form that only a few malenti survive childhood. Those who do are capabale of greater evil still...
A later tale- I think this was composed during my first AD&D campaign when the PCs had their first underwater adventure- which meant they were mid level. Again the AD&D roots are very obvious- in 2nd edition Aquatic elves lack certain innate elven magical abilities, including immunity from sleep and charm spells, and among the Sahuagin descripiton in the MM one finds the curious malenti...