It's not an oversight, just a different way of working with the aggravated damage system. Let's dive into it!
To start with, the Strength knacks that inflict aggravated damage - Divine Wrath for grappling, Disfiguring Attack for punching and Hurl to the Moon for throwing - are all available to a Scion from day one with no Legend restriction. Anyone of Legend 3 or above can use them with impunity (Scions of Legend 2 can also technically get them, but they aren't yet capable of paying the 5 Legend to use them), allowing them to start wrecking it up with agg early in their Hero careers. In contrast, Mortal Stroke is a level five boon, which means that no one has access to it until they're Legend 6, well established as Demigods. Unless your game is starting character creation at Legend 6 or higher, Mortal Stroke will never be a "replacement" for other methods of doing agg simply by virtue of being unavailable. That much higher Legend restriction is also one of the reasons it costs less, because Scions have to wait so long and invest in so much War to get it.
More importantly, however, it's a different kind of cost from the Strength knacks, which means that it raises different concerns. The fact that using Mortal Stroke requires an action makes it significantly slower to use in combat, as opposed to the Strength knacks which can all be reflexively activated every time a Scion hits her enemy and don't need a separate five ticks to arm beforehand. Depending on the length of the combat, the abilities of the enemies and what the Scions are trying to achieve, that five ticks (or ten or fifteen or twenty, depending on how many times a Scion needs to use the boon) may be the crucial deciding factor between different outcomes. Scions can get around this sometimes by using Mortal Stroke on a weapon before combat starts to have it ready, but this only saves the first five ticks, and still requires them to drop another action every time they want to re-agg their weapon. That's a pretty hefty restriction in comparison, which is why the Legend cost is so low.
Also, Mortal Stroke costs a point of Willpower, which is a different pool with different possibilities and consequences, which makes it more comparable to the cost of the knacks. While Willpower is easily refilled by Charisma knacks from helpful bandmates, it's a much smaller pool and needing people to refill it also ropes them into having to spend extra actions in combat, which may or may not torpedo other things they need to be doing. Other combat powers also use Willpower, putting it at more of a premium in those crunch situations when people may not have the time to refill it for you, and using large portions of your Willpower pool inevitably puts you at much higher risk of Virtue Extremity, which can rear its head at the worst possible times (especially in combat, when people, objects and environments are in danger of being damaged by the fallout and pissing off any number of Virtues).
| Pros | Cons | |
| Strength Knacks | Reflexive Available to all Legend levels | Legend-expensive Usable only by self |
| Mortal Stroke | Legend-cheap Usable on others as well as self | Requires an action to activate Willpower-expensive |
So this is all intended, actually. The two are meant to be balanced against one another, the abilities of those with great strength stacked up against those with the battle acumen to bolster themselves and allies so that both are valid ways of getting there but have different difficulties attached.
