Unlike modern "stalker" mods, the classic 1.2.5 experience was designed to be disruptive and terrifying through unique environmental changes and direct attacks:

To begin the haunting, players must build a specific shrine using gold blocks, netherrack, and a special "Herobrine Totem" block.

Once summoned, the world begins to change. You may find leaves stripped from trees, random 2x2 tunnels in caves, and mysterious sand pyramids in the ocean.

Herobrine doesn't just attack; he messes with your mind by placing redstone torches, spawning eye-less animals (cows, chickens), and occasionally teleporting behind you.