The Crusade is a set amount of stages that can be completed every 2 days by level 40+ heroes.
The health of heroes does not reset during a crusade, and any that die during a battle will be eliminated from continuing until it resets.
The difficulty of it will scale depending on the power of your heroes.
Each stage will reward Crusade Coins and Battle Points. Certain stages will also unlock Chest Rewards with even more loot, and others can also grant access to a temporary vendor that can sell items at a discount.
Crusade Coins can be spent at the Crusade shop.
The items for sale here are hero fragments and 5-star specific heroes.