OMG, first encounter I had with one in the wild, I had my Doublade. It took 3 hits and once it went down, it took down my Doublade with it... Still not sure how that happened because I kinda didn't pay attention to the battle
My guess is that this Trevenant has used the attack "Destiny Bond" on your Doublade. This attack means that once used on an enemy, if that enemy takes the user down, that enemy will automaticly faint too. After you took it down, your Doublade was taken down too because of it.
yay, I figured that's what it was, but it just surprised me because I looked at my game, one instance I see a wild Trevenant, I use Night Slash, I look away for about 3 seconds and when I look back, my Doublade goes down