Fëanáro passed much time in shock and despair.
But, being Fëanáro, his thoughts eventually turned from grief at having pledged his life to a cause so lamely... to REVENGE.
How could he make the Lord of Mandos suffer as he himself had suffered? First that oath- "On the House of Fëanor the wrath of the Valar lieth from the East unto the uttermost West, and upon all that shall follow them it shall be laid also..."
Wait.
Fëanáro's non-existent heartbeat slowed as he realized.
Could Námo have made that foolish a mistake?
Fëanáro went over it again in his head. YES, he had!
Instantly he jumped up (as much as a disembodied fëa could) and went straight to see Námo.
"Ah, Fëanáro," Mandos said, looking down his nose at him. "Are you recovered, my dear child, from the pain of your epic fail?" (Okay, so he was gloating, but seeing as how the Firstborn before him had been the cause of every single one of Námo's headaches since the Two Trees were felled, Námo thought he quite deserved to gloat.)
"Yes, my lord," Fëanáro said simply, with an undercurrent of smugness. "And have you gotten over yours?"
Námo's face turned beet red. "Insolent Noldo!" he boomed in his Doom-of-Mandos voice, which was a bad choice because it only made Feanaro burst out laughing.
Seeing this, Námo became... a little bit nervous. Fëanáro was- as much as he hated to admit it- very intelligent; was there a chance he had figured out something that could damage Námo's carefully cultivated Doomsman image if someone else found out? "...What do you mean?" he asked, trying to still sound regal and failing.
"Your Doom," Fëanáro explained gleefully. "You told me, 'On the House of Feanor the wrath of the Valar lieth from the East unto the uttermost West, and upon all those who follow them it shall be laid also.' You then went on to list a myriad of punishments and consequences for my most noble house and its followers... only, you didn't."
"What do you mean, insolent Noldo?" boomed Námo, but less loudly than before.
"Well," Fëanáro explained, acting every bit the condescending professor he had been to his seven sons in Tirion, "a pronoun such as "them" describes the most recent plural noun in the sentence. "The House of Fëanor" is a singular, albeit collective, noun, and not plural. Can you tell me what the only plural noun preceding the word "them" is?"
Námo, the Lord of the Halls of Mandos and Doomsman of Arda, went deathly pale.
"Say it," Fëanáro commanded gleefully, relishing in being the one to gloat for a change.
"The... Valar?" Námo murmured, his voice actually squeaking.
"Exactly," Fëanáro proclaimed. "My dear lord Námo, you just doomed yourself, all the Powers of Arda, and every last one of your followers."
The most dignified reply the great Námo could manage was "BRAWRRRRRAAAARGHHHHH!" With that he went screaming and wailing in embarrassment out the door of Mandos (giving Fëanáro the chance to escape before one of Námo's Maiar lured him back inside with a piece of shiny silver jewelry), shooting straight across Ekkaia and into the Void.
It took all thirteen other Valar (and promises of regular beach vacations from Manwë and much encouragement from Vairë) to talk him back out again.
And that is why the Edain given Númenór fell and were drowned, the kingdom of the Faithful who escaped Númenór splintered within years, Frodo the savior of Middle-earth was wounded beyond repair in Middle-earth (Gandalf so totally knew what he was getting the poor Hobbit into), and the Ainur and Eldar of Aman had to deal with three hobbits and a dwarf for nearly three centuries. Not to mention that Námo had to deal with two more Kinslayings right on each other's heels, meaning large quantities mortal enemies in the same Halls, six sons of Fëanáro, and an extremely annoyed Vairë.
(As for Fëanáro, the Valar couldn't decide whether or not to punish him for stating the truth, so they just ignored him, contenting themselves with visiting the Halls and saying, "Have you seen any other bright Valar?", "So-and-so is acting like the brood of Morgoth," and other reminders of Curufinwë Fëanáro's epic fail.)
