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As in everything else they face the battle together. Had he his druthers, Fili knows not whether he would prefer Kili safe inside the mountain, or at his side. In the end there is no choice- where Fili goes, so does his brother.
He need not look left to know that Kili is there- predictable as the rise and fall of a smith's hammer is his brother at his side. Inseparable as children- since the birth of the younger, and inseparable now- complete only with the other at his back. Home is where his brother is- whether a mountain filled with gems, or a small forge in the land of Men. They are two sides of the same coin- gold and obsidian- one soul split between two bodies.
When Thorin falls Fili speaks neither grief nor intention, for his brother knows without words. They turn as one and rush headlong into the fray, guarding their downed Uncle- King!- with every ounce of strength they have left to call on.
Words murmured by their mother long ago come unbidden into his mind: You are Thorin's heir, but first you are Kili's brother. Watch for him as he will watch for you.
The spear comes out of nowhere and takes Kili in the side, severing skin and bone, and pinning him to the rock face. His Uncle's fall no longer resonates and all else stills as he rushes to his brother's side. Kili lets out a mewling cry as their eyes lock, struggling to speak words that will not come. Fili knows the words as well as he knows his own grief and responds: I am here, brother. I am here. Do not leave me.
Dark eyes roll back and breath slows, even as his own heart beats faster- breaking in his chest. Fili feels the arrows piercing his armor, but the pain is nothing compared to the fear blackening his soul. A score of prayers leave him- to Mahal, to the Seven Fathers, even to the mountain itself- though he knows well there will be no answer. Save my brother. Let him live. Blood fills his throat and chokes off his prayers.
He presses their foreheads together, tears washing away the blood and rot of battle. I am here, brother. I will not leave you. Rest now. It is not until the final breath leaves his brother's body that Fili closes his eyes. Forgive me, brother. Wait for me in the Halls. I am coming, Kili...
This is how the world ends- not with a bang, but a whimper... and the ghost of a brother's name falling from his lips.