Orihime Inoue

[Omni]

"Commander Lexa’s rapier, a weapon of old and her go to weapon for combat. Fine flexible material, good for stabbing or slashing and he very hilt of the blade is a formable foe as it was forged form the deadliest of materials from the ground, undergoing nuclear warfare. Its bite leaves a radioactive poisonous effect on anyone other than Lexa herself and with each blow her opponent is further and further intoxicated causing the onset of blindness, paralysis, and excruciating pain.

At first contact with Trigedasleng one would experience a euphoric, ‘washing over your body’ feeling that would leave your stomach in knots. This is uncomfortable at best, but not harmful. Two or three strikes later the knots would start to become greater, but still nothing that would cause any handicap.
By the 5th or 6th time struck by Trigedasleng, your vision would begin to lose focus and a few strikes later become blurred and hard to see. At this point the effect is upon you and will not diminish on its own so long as you are within the presence of Trigedasleng (60 Meters).

Once the vision has become a blur, further exposure (8th to 9th strike) would result in a damage to the nervous system causing difficulty moving. Any further exposure to the weapon at this point results in random surges of inexplicable pain.

The effect Trigedasleng has on Lexa’s opponent is that of a foreign agent attacking the body, such as radiation. Due to the nature of this attack Lexa’s opponent’s body can adapt and create antibodies causing them to be immune. This means that if she reencounters the same opponents, over time through multiple fights, they would become resilient to the effects of Trigedasleng. Well, at least for everything except the hacking and slashing that is."

I hope this works, just let me know!

The "immunity" thing would only be flavour if you wanted to go with it. It's not the kind of balance we go for here, that it would give an advantage on the first fight and not on subsequent ones.

So the move would require another downside. If you make it so that the move doesn't do much actual damage besides the radiation effect that would be fine, and/or the opponent can slowly get used to the effects over the course of a battle. You'd need to elaborate on the specifics of "difficulty moving" as that'll affect the balance.