When a Max-Level Expert Gets the Cannon Fodder Villainess's Script

What happens if you're locked up with a mute for three hundred years? Cang Yao can tell you from personal experience: you die! And what happens if that mute suddenly starts talking one day? Cang Yao can tell you from personal experience: you wish you were dead! Cang Yao, the immortal, undying Demon Lord who terrorized the four seas, was sealed away. Sealed with him was a yellow-haired girl who was nothing special to look at and as thin as a bamboo pole. - The Twelve Bone Caves were cut off from the sun, with no changing seasons. Chains bound Cang Yao's limbs, and a beam of holy light shone down on his head. The moment he even thought of breaking the seal, countless bolts of lightning struck him. Each strike deepened his hatred. He swore to kill every cultivator, starting with the person sitting cross-legged nearby, eyes closed and endlessly chanting scriptures. - Day One of the Seal Cang Yao: “Little girl, don't you want to get out? Are you really content to stay sealed in here?” Xu Qingyan: “...” Day Two of the Seal Cang Yao: “So what if you sacrificed yourself for the immortal sect? They'll forget you sooner or later!” Xu Qingyan: “...” On the third day of the seal, before Cang Yao could say a word, Xu Qingyan pulled out a huge stack of scripture books, sat cross-legged, and began to read them aloud. The sound made Cang Yao even angrier. He wished he could break out and kill her. After three hundred years of loving and hating each other, they'd thought they'd never see the light of day again. Then one day, the seal suddenly began to weaken, and the Demon Lord broke through and escaped. (A devilishly charming, arrogant, cool, and domineering male lead vs. a disciplined, meticulous, old-school heroine)