I say mana-moon
Like, you know, the moon.
Japanese words are almost exclusively made of constanant-vowel syllables. So it's pronounced ma-na-mu-ne
It is based off the Japanese pronounciation. Mu is pronounced Moo.
Its mana and m-ew-n
It's a play on words of the sword "Masamune"
It's pronounced Ma-na-mu-nay
Assuming Japanese reference from Masamune, it would be Mah-nah-moo-nay.
In Japan, baseball (which is very popular) is pronounced "base-u-ball-u" as that's more akin to something in their language, and easier for them to say. Say it how it feels comfortable.
© 2014 Riot Games, Inc. All rights reserved. Riot Games, League of Legends and PvP.net are trademarks, services marks, or registered trademarks of Riot Games, Inc.