Simplify the following text for a Grade 5 native English-speaking person.
Original: In Tolkien 's Middle-earth , mithril is extremely rare by the end of the Third Age , as it was now found only in Khazad-d Ã '' m. Once the Balrog destroyed the kingdom of the Dwarves at Khazad-d Ã '' m , the only source of new mithril ore was cut off .
Simplified: Mithril is very rare and precious . It was only found in Khazad-d Ã '' m ( Moria ) , where it was mined by the Dwarves , and ( perhaps ) in NÃ menor .