Here's a more detailed and immersive rewrite of your passage:

Much to Tom Riddle’s frustration, he had been forced to travel through time—thirty-six years into the future, from 1943 to 1980. It was not an impulsive decision, nor one made lightly. This journey had been a necessity, a calculated move in his relentless pursuit of immortality. One final piece remained to complete his collection of Horcruxes, and the answer lay not in his own era, but in a future he had yet to shape.

Manipulating people was second nature to him, and securing the cooperation of Horace Slughorn had been almost laughably easy. Through carefully woven lies and forged records, Riddle had been seamlessly introduced into Hogwarts under a fabricated identity—a transfer student, arriving from an unspecified wizarding academy abroad. Few questioned it. Fewer still dared to scrutinize the details.

His sorting into Slytherin had been conducted in private, away from curious eyes and inquisitive whispers. The. Clad in Slytherin’s modernized robes—subtly different from those of his own time but still bearing the serpent’s insignia—he assessed himself in the mirror. The fabric was finer, the tailoring sharper, and he found the updated design strangely appealing. With a practiced hand, he smoothed his dark hair back, ensuring every strand lay in perfect order. His reflection revealed nothing of the deception beneath his composed exterior, only the poised elegance of a young man who belonged.

Stepping out of the dormitory, he moved with deliberate ease, descending the stone staircase toward the Great Hall. The corridors of Hogwarts felt at once familiar and foreign, steeped in the same ancient magic yet altered by the passage of time. He walked among the students, slipping effortlessly into their midst, just another face in the sea of unwitting pawns.

Today was not simply the beginning of a new term for him. It was the dawn of his greatest conquest.