The country Rostorl has lost a war against Dyneskal Empire, the most powerful nation of the Vyashion continent. Balor, the emperor of Dyneskal, aims to expand his domain even more. However, he is told that he will die by the hand of his own offspring. He orchestrates the death of his own son, but his grandson Areus is saved by his elven mother. Areus grows up and swears revenge against his grandfather, attempting to become the country's most powerful gladiator to achieve his goal.
Trinity: Souls of Zill O'll is a prequel to Zill O'll, depicting the events that led to Balor's demise. Unlike its predecessor, it is an action role-playing game. The player controls a party of three characters - Areus himself, the female warrior Selene of the vampire-like Darkenith tribe, and Dagda of the Boldan race of giants. The player can directly control only one character at a time, but it is possible to switch between them instantly and initiate combo attacks that way. The three can also perform a powerful attack together when certain conditions are met. Battle environments are destructible and spells physically affect the entire area rather than just the characters.
Like in the previous game, sub-quests are available in town guilds, and the player can opt to tackle them in a different order. The character affinity system, however, is not present, and the player is unable to create an own character or choose between different initial scenarios.