The Clear Bell and Tidal Bell are used in order to obtain both of the Tower Duo Pokémon, Ho-oh and Lugia respectively. When placed, the bells resemble that of a spherical bell's outer shell. The Clear Bell has a distinct golden color, while the Tidal Bell has a more white appearance.
Before this process can begin, you must first obtain the Clear Bell or Tidal Bell, depending on the legendary Tower duo you want to spawn. For Ho-oh, the Clear Bell is required and for Lugia, the Tidal Bell. After obtaining one of these bells from a Minecraft structure chest, you must place them on the underside of a block.
After the bell is placed, each dawn of the Minecraft day cycle, the bell will have a 1% chance to begin ringing and glowing. When the bell activates the bell will have a guaranteed spawn for its respective legendary when the day reaches dusk. The legendary will seek out and try to attack the player who placed the bell, first. If this fails, such as the player not being online or already in a battle, the legendary will then spawn from the bell and wander around the bell's location. On a successful spawn, any bell that rang will be destroyed and lost.
Since 7.x.x. Each bell will spawn a legend. If 5 bells are nearby(distance set by bellInclusionRange in config) all 5 will spawn a birb. Additionally, having multiple raises the chance of the bells ringing. The formula is
bellSuccessChance * number of nearby bells
This means by the default settings of 0.01 success chance, 100 bells placed near each other would guarantee 100 legends to spawn at Dawn no matter what. This can be combated by lowering the success chance. Alternatively, bellInclusionRange can be set to 1, this way bells are independent and the chance is similar to old methods.
"bellInclusionRange": The range in which a ringing bell will seek out other bells.
"bellSuccessChance": The chance per every dawn that a bell will begin to ring.