If not % Health True Damage, then what?

A lot of people complain about % health true damage being unfair and having no counterplay. Which is only about half right. While, yes, there is no way to directly itemize against % Health true damage, there doesn't necessarily need to be. That kind of damage exists to serve a very specific purpose to balance out a champion's kit in multiple situations, and does a good job at it.

Vayne's true damage is the one that sees a lot less publicity due to her not really being in the meta right now, but whenever Vayne is popular, it's immediately touted as proof of her imbalance. The thing about Vayne's true damage is that it only happens on every third auto attack from vayne and deals 12% of her target's max health. But how much damage is that, really?

Against a champion with 6000 health, which is a rather incredible amount to have, Vayne does 720 true damage. This sounds like a lot, but that's only every third auto. On average, Silver Bolts adds 240 damage to each auto attack. When your auto attacks are all critting for 700-1000 damage each, even 100 armor isn't enough to make silver bolts half of Vayne's damage output. But let's assume the tank that Vayne is trying to kill has 200 armor. Each auto will deal 233 physical damage (assuming ~700 damage before mitigation) and 240 true damage, which means it will take 12 auto attacks for vayne to kill her target, which would realistically take 8 or 9 seconds of uninterrupted auto attacks to actually pull off.

"Why should vayne be able to kill a full tank in 8 or 9 seconds even with those ridiculous tank stats?" You may ask. Well... why shouldn't she. Vayne's auto attack range is shorter than a vast number of CC abilities, Vayne is unable to switch targets in between 3-attack intervals due to the way silver bolts works without losing damage, and 8 or 9 seconds to kill your target is an eternity in the middle of a full-blown fight. Vayne has to put herself in a massive amount of risk to do that damage, and the player needs an incredible amount of skill to survive that situation, so why shouldn't they be able to kill any tank if they can pull that off?

Something similar is true for Fiora. With a good amount of AD, her vitals-strikes can do ~15% of her target's max health in true damage, meaning it takes 14 vitals strikes to kill her target. But, again, there's no reason why she SHOULDN'T be allowed to kill her target if she hits their vitals 14 times without them healing or getting a shield.

Fiora and Vayne are the only instances, right now, of % health true damage. They both have their true damage tied to a little minigame that encourages active involvement in a team fight, and that's not a coincidence. Both of those champions are designed to have little "mini-games" that guarantee that they can kill their targets (as they are assassins after all) regardless of what their target may be should they consistently play the mini-game well.

Which is why the damage kind of has to be % health true damage. If the damage was either physical or magical, then you run into a balancing nightmare. If the damage is too high, then Fiora or Vayne have way too easy of a time just popping squishy champions, changing them from Duelist-carries into pure assassins. If the numbers are too low, then the mini-games begin to be useless against tankier opponents; if you're trying to kill someone with enough resistances, it becomes pointless to bother wasting your time focusing on managing the extra damage in your kit because you're not actually gaining much for doing so; instead, you're just killing time until an opportunity to focus a squishy opens up. Which is the key point; Fiora and Vayne aren't about insta-popping squishies (compare their burst to any real assassin's if you don't believe me), they're about being able to win any 1v1 fight with proper management of their resources so that, with good play within a teamfight, they can take out the entire enemy team one by one, which is the entire point of an AD carry.

"Why do Fiora and Vayne need % health true damage if no other AD Carries do?"

Because other AD carries have more range. The only AD carries that don't have significantly more poke-ability, and therefore ability to consistently deal meaningful damage to the enemy team without needing % health true damage, are Quinn, who is more commonly seen as a pick for a mid-lane roaming assassin rather than as a traditional AD carry (and even Quinn is designed to be far more focused on Squishy-popping than Vayne or Fiora) and Graves, who is a top-lane and jungle pick who builds like a bruiser and acts like one, too, rather than being a traditional AD carry.

Once you make a champion have a small amount of range, the entire dynamic of how they are able to win a team fight changes. If a champion has enough range to sit in relative safety outside of a team fight like Tristana or Jinx or Kog'Maw, the rest of the team can make Tanks a non-issue. If your team can CC the enemy frontliners enough that you can stay out of their range, then you don't need to be able to kill the tanks nearly as efficiently or quickly; you have all the time in the world to just wail on them. Fiora and Vayne can't do that; you can't keep Malphite or Darius away from Vayne without also keeping your damage dealer away from the enemy team, and the same goes for Fiora.

But % health true damage also serves another purpose when properly tied to a mini-game-like mechanic; it prevents champions from turning into a ball of stats. Since so much of Fiora's damage relies on hitting your vitals, she's never beating you entirely because she had more stats than you; the fact that her main damage source is telegraphed means it is something both you and her can play around. Unlike champions like Jax or Irelia, where the counterplay to their kits is "Don't fight them if you can't beat them," champions with huge bursts of telegraphed damage have additional counterplay options. Since Vayne can't switch targets in the middle of building up for her silver bolts, you can try to manage your positioning such that she either has to switch targets or has to dive into your team to continue chasing her original one. Since Fiora's ability to hit vitals is heavily tied to her ultimate, you can force her to stay inside of your team where you can destroy her with your own damage and CC. Both Vayne and Fiora have ways of playing around your attempts to play around them, such as Tumble, Vayne's Passive, Lunge, and Riposte, but that just means that both you and the champion with true damage have some kind of input as to whether or not they can kill you.

Now, before anyone gets the wrong idea, I'm NOT saying that Fiora and Vayne are always balanced. Fiora looks a little on the strong side (a lot on the strong side) and her passive may need more of its damage shifted into the scaling rather than baseline damage; however, there is a common conception that just because one specific part of their damage output can't be specifically itemized against, that specific part of their damage output is, inherently, "Broken" and needs to be removed, which just isn't true.