Huh, interesting decision, I wonder if really only one class will be able to do that on release.
I get why they did it, that tadpole use is the only one that I felt ever tempted me as a good player
Why? it takes like 30 extra seconds to get the rune anyway.
Now you have to use a tadpole when you plug in the rune. Meaning you can't save Shadow and stay pure unless you are a barb
It's hard to say for sure but I would be surprised if total purity has an impact on the story. As long as you avoid using it as much as possible, having only used the tadpole once or twice will probably not matter. My guess is that becoming branded a True Soul is probably the point where having used the tadpole changes your game.
