It consists just to have a menu before a battle (with 1player vs 1player) that provide to chose a music only during the battle, these musics would be added in the mod
We could also with the trainer editor choose a music battle (that would be load during the battle) on a trainer npc that the player could not change (like in pokemon games).
I have two examples to explain that:
1. It's exactly like when we add a music file in a ressource pack and we do in game at the beginning /playsound (music name) and at the end /stopsound (music name) (it works with every sound we choose).
2. For that I use the exemple of the system npc's skins already present in the pixelmon mod , there is some skins basically present in the mod but we can add somes manually in the mod .
I know only players with that mod can see these skins (internal files it is called no?) but it's not a problem, the server have just to make a download of this mod on his website.
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I hope it will interest you, I think it could be a great feature to be add and it would be very useful for servers who want a great ambiance ^^ .