While this doesnt sound like much, the Pokemon in this game dont have much Retreat Cost to begin with, so reducing it by one is just as good as being able to swap out for free
If you attack with it, more card advantage
Furthermore, if their trainer knows that their Beheeyem BREAK is about to get knocked out, they can send all the energy attached to them to benched pokemon to prepare for upcoming tactics while also slamming the foe with 100 damage
Mirror Room is basically Mirror Image in room form, which is a nice follow-up to the more expensive room, or a fine value play that bridges the gap until you can pay the full 7 mana on Mirror Room // Fractured Realm
It's a very lethal creature and is the reason this saga makes the list
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