Selene's core issue is that Eos's regen and heal potency up is so OP, but I don't think giving her a debuff skill really fits the idea of Selene. Wind hitting 5% to equal AoE would be nice, or shorten it's CD so we can give it more uptime.

But I agree we need to replace Silent Dusk entirely. I like the Vul Up idea, but my suggestion would be a skill that guarantees the next Galvanizing skill crits. It would be a great counter to Eos's Whispering Dawn, and would synergize and absolve some of the issue with our sheild's being a smaller flat rate compared to AST Noct skills.

It would need a long CD (120 seconds like Dawn, maybe as high as 180 like Benediction), but it would give SCHs the same sort of choice of style AST's offer, and actually give us a reason to utilize Selene.

Want to focus more on instant casts with steady imput? Take Eos. Want to focus more on mitigation? Take Selene.