This is my attempt at FFIX's long-standing Thunder Slash Bug.
Steiner's version of Thunder Slash is broken because it would almost always miss unless against heavy monsters, Beatrix's isn't. Why is that? The finger sometimes is pointed to a theory where Iai Strike has too much accuracy (let's face it, it could stand a nerf) and TS has too little, meaning Iai Strike "steals" it's accuracy. However after a look-see, this doesn't appear to be the case. TS was accidentally given the wrong damage formula, running off of Gravity Magic (Demi, Aqua Breath, etc.) instead of Magic Weapon. The evidence for this is in the data surrounding it: outside of damage formula and different power/MP levels, Thunder Slash shares next to no difference from the rest of his direct damage skills. Including no inherent accuracy (because weapon based moves have their accuracy overridden by your usual attack accuracy) which was why it was always missing. Beatrix's would work because it's a completely different ability with other Monster ones and is a Magic Attack. Go figure.
So what does Thunder Slash do now? Thunder Damage to one enemy that is more than Stock Break, but slightly less than Climhazzard and much less than Shock. It runs off of the same damage formula as Climhazzard, so it hits Magic Defense with Physical Attack. Yes that is a load of power but it's also normally encountered late-game anyway. There was once a patch that would change it to a bog-standard Physical Strike, but it turns out that formula robs TS of it's element due to running off of the weapon's element.
Standard PPFs here, apply to your discs and go. Oh, and this is for the US 1.1 version (the one with the Square Enix logo.)
We have EVEN MORE themes now, but some old ones are gone...