Endless Graveyard is a special type of mission in the game that allows you to battle Zombies to collect Shovels. To access this mode, open the Missions menu from the main game screen and select Endless Graveyard.

In the newly opened window, you can see the available Boss Zombie to attack, its strength, and the rewards for defeating it.

To attack the Zombie Boss, tap the Challenge button at the bottom of the screen. You will need 1 Graveyard Key to initiate the attack.

Before fighting the Boss, you will need to fight several waves of zombies, each one stronger than the previous one. If you are defeated in one of the waves, you will not be able to move further and will have to start the same Challenge again.

After engaging in several battles against zombies, you can either continue defeating stronger enemies to improve your rewards or claim the rewards for the Zombie Bosses you have already defeated.