You choose from one of three Overlords to train at the start. You have a set amount of time to train and prepare for the fight against trillion to defend the underworld. You can train stats, learn skills, go into a dungeon to gather items, strengthen weapons, and rest to get rid of fatigue. You can also interact with the Overlord to raise affection points, raise affection to a certain level and events related to the Overlord will occur.
Affection points are also used in the battle against Trillion where they act as a buffer for your health and mana. As long as you still have affection points in the battle you will also have the option to retreat from the battle, giving up a section of the underworld for more time to train. If the Overlords health reaches 0 they will be able to activate a final death skill that could deal massive damage, seal one of Trillion's attacks, or stop Trillion's advance for a longer training period. The Overlords items, equipment, and seals will be passed on to the next Overlord you train along with a portion experience.