Find Altars scattered about your world dedicated to the worship of allmighty deities. Either amass a following or become clouded by the delusions of the Moon, Sculk, Nether, or End; your choice ;)
Altars for the Whispering Myth can be found in Ancient Cities

Alternatively, can be crafted like so:

Altars for The Lunar Vigilance (moon), Blazing Harbinger (nether), and The Heart of the Ender (end) are currently not available.
Once obtained, you can right click to bind with your deity and begin your ritualistic practices.
Once binded, you cannot destroy the altar without angering your deity and starting a Severance period. (currently not available)
You can offer items your deity may like by right clicking the altar with the item in your hand. Most items will return nothing, but a handful will return a good reward.
New Resources and Gear:
All of these can be obtained through offerings, however alternate ways to gather them are provided.
Sculk Covered Rocks: Throwable, can be obtained through ancient city loot.

Echo Cores: A crafting ingredient, can also be crafted by surrounding a diamond with echo shards.

Sculk Covered Iron: New armor and gear slightly better than Gold, can be crafted by surrounding any iron tool with sculk veins or obtained through ancient city loot.

Whispering Steel: New armor and gear slightly better than Netherite, can be crafted. (whispering steel ingots only obtainable via offerings)

Tools can be crafted by using bones instead of sticks, and armor crafted with echo cores in the center. (Boots use none, chestplate uses two)

WHISPERING STEEL!
The Riftcaller: New weapon. Only 1 can be obtained in a world. Cannot be crafted.

In order to allow for more than yourself to use an altar you must offer a bottle of blood to your deity. In order to get your friend's blood, have them use a scalpel (recipe below) with an empty bottle in their offhand to collect their blood.
