The Darkening of Tristram

The start of the Diablo's story.

After many years, Leoric established himself as king of Khanduras and brought with him the Zakarum faith. He made Tristram Khanduras's capital, with his seat of power in the ancient Horadrim monastery, now a cathedral. The Archbishop Lazarus, Leoric's most trusted adviser and an envoy of the Zakarum, ventured deep into the cathedral, attracted by some mystic energy. At the bottom he found Diablo's soulstone.