Darkside should keep draining, as we already have a static gauge in the form of the Blood Gauge. I'm mostly aiming for Darkside to not be so set and forget, which in result changes up the flow of DRK and makes it feel like less of a WAR clone and more of a unique tank that's managing its buff timer, when compared to GNB's cartridges (and heavy GCD burst), WAR's current gauge work, and PLD's sustain (which will soon be something else entirely). This also changes Edge from "use on cooldown" to a tactical choice to keep your Darkside duration up. Abyssal Drain would replace it as the go-to for damage, as it'd also give a small sustain heal to help and would prevent overcapping on Darkside when MP needs to be spent.
