Mythril

Description Might be of some help to you...
Found in Old man's house
Compare Mythril to other Key Items
Items of Interest: Mythril – 1 shown
Item Type
Whether this item can be found or collected multiple times
Repeatable		 Area Screen Details
Mythril Item Yes Old man's house Old man's house Talk to the sleeping man when your total battles end in 11, 33, 55, 77, or 99 (repeatable until you get the Gold Armlet)
Black Materia Snowboard

Related

Mythril

Found In Semitt Falls B5
The Mythril is a nice Key Item, for two reasons. First, it allows you to start buying Mythril arms, which are quite good. And second, it's a rare Key Item that actually disappears from your inventory once you use it. Huzzah!

[Mythril]

Hilda teaches you the [Mythril] keyword after you return from Fynn with the Ring.