You find yourself on an island, the land of the undead. The only way to escape is to find and enter the yellow sphere. But be aware - it is protected by skeletons. You can attract zombies to fight against skeletons. During day, zombies are your friends and will follow you. But during night they become evil and fight you!
The strategy is to explore the map and find zombies during day and guide them to skeletons so that they fight each other. Use the radar (upper right corner) to identify the goal sphere (yellow), zombies (turquoise) and skeletons (red).
Reaching the goal sphere gives you one star, eliminating 25% enemies (zombies+skeletons) gives you two stars and you will get three stars for elemiating at least 75% of the enemies.