I think something like this would have a relatively longer cooldown, no? Duration and cooldown are hard for us non-devs to actually balance. I know from my modding experiences, even if we think we can balance those, trial and error is the only way to truly know if it's "fair" but if I were modding this game, I'd start with a longer cooldown. Something around 90 seconds. It could have an additional "only available under the effects of x" to make it more understandable why it's a consuming ability and not just some trait. You can only turn Oblation/Dark Mind into a HoT every 90 seconds makes it more understandable why it's not just a trait so the heal isn't coming every 30s from Oblation (recharge ability cooldowns are recharge time divided by charge count, so 30s cooldown on Oblation in "perfect" rotation)
Again, just a morning coffee idea that's probably a waste of energy to actually think about lol.