Spirits can be used at arena
You can drop some spirits to arena, and they just will fly there doing nothing, which is an obvious outcome. However, it still counts as a fight, and your creatures come to arena to watch the "battle". I think it shouldn't be allowed, just like imps are not allowed to be dropped to arena.
Game Version:
Steam Public
Platform:
Windows
Can replicate. It's nothing game-breaking because the beast minions can not attack (and thus gain EXP) from the spirit, but it's still mildly confusing.