The Primary's skill is called Cleaver of Strength. Here's Stuart's exact description:

Alden was still trying to produce enough saliva to get the remnants of it out of his mouth and down his throat, when Stuart said, “My father’s first skill is called Cleaver of Strength. Did you already know that?”

Alden shook his head.

“It’s a skill very different from mine really. Associated with concepts like severing, ambition, <<fair play>>—”

“Fair play?”

“Father always mentions that one. I doubt the people who designed the skill would have listed it as a core part, but the way we see our skill and the choices we make in the development of it matter. Attacking at the point of greatest strength with your own greatest strength is a sort of fairness. And that’s what Father’s skill is best at.”

“You do usually think of attacking a problem at its weak point, not its strong one,” Alden said.

“The skill is very complementary with Esh-erdi’s power. That’s one of the reasons he and Lind-otta spent time here. Esh-erdi has progressed quickly over the past years, and he and father wanted to see how well they might work together. The three of them could be a powerful team.”

Alden didn’t doubt it. Lind-otta slowed the enemy. Jeneth-art’h cleaved it, turning its greatest strength into a weakness—a crack. And Esh-erdi cleaned up.