https://universe.leagueoflegends.com/en_US/story/champion/yorick/
From his Bio on Universe:
"One morning, as he tended to his duties in the cemetery, Yorick was interrupted by the sight of a pitch-black cloud roiling across the surface of the Blessed Isles, devouring everything in its path. Yorick tried to run, but the cloud quickly enveloped him and plunged him into shadow.
All around Yorick, living things began to writhe and contort, corrupted by the foul magic in the Black Mist. People, animals, even plants began to transform into vile, ghoulish mockeries of their former selves. Whispers emanated from the turbulent air around him, and his brothers began ripping the vials of healing water from their necks, as if the objects were causing them great anguish. A moment later, Yorick watched in abject horror as the monks' souls were ripped from their bodies, leaving cold, pale corpses behind.
Among the quieting screams of his brethren, Yorick alone could hear voices within the mist.
"Remove it. Join us. We will become one."
He felt his fingers grasping for the vial at his neck. Mustering all his resolve, Yorick forced his hands away from his throat and commanded the howling souls to stop. The Black Mist writhed violently, and darkness overtook him.
When Yorick awoke, the winds had calmed, and the once-fertile lands had transformed into the grotesque hellscape of the Shadow Isles. Isolated tendrils of the Black Mist clung to him, trying to overtake the one living thing not yet corrupted. As the Mist wrapped itself around him, Yorick saw it suddenly recoil from the vial at his neck. Yorick clutched the blessed water, realizing it was all that kept him alive.
In the days that followed, Yorick scoured the islands for survivors, but found only the twisted remnants of what once lived there. Everywhere he walked, he witnessed wretched spirits rising from the bodies of the dead.
As he searched, Yorick slowly pieced together the events that led to the cataclysm: A king had arrived seeking to resurrect his queen, but instead, had doomed the Isles and everything on them.
Yorick wished to find this "Ruined King" and undo the curse he had unleashed. But he felt powerless in the face of the seemingly endless death that surrounded him.
Almost lost within his grief, Yorick began to speak to the spirits around him, attempting to find solace with them as he had as a child. Instead, as he communed with the Mist, corpses left their graves, guided by his voice. He realized the bodies he once laid to rest were now his to command."
So it can kill, and talk to people to get them to take off their protective vials of sacred waters. He's also had an ability from young to communicate with spirits, which later evolved through this to full necromancy. He's continually resisting the influence of said Mist.
