Leviathan is a new Eikon option in the Final Fantasy 16: The Rising Tide DLC. You will gain access to both the summon and its abilities near the beginning of the DLC’s story. Investing in these can help both during the campaign and returning to the main game.
The Rising Tide is accessed very late in Final Fantasy 16. You will have to complete the main story quest “Back to Their Origin”, which is the penultimate quest right before the final boss and ending. This means you won’t have much time to play around with Leviathan’s latest skills.
Final Fantasy 16: The Rising Tide introduces five new skills for Clive. There is no restriction on what you can purchase. If you previously farmed ability points, you can max out all options immediately. However, if you lack AP, you should choose each skill conservatively, based on your playstyle.
While the new abilities are great for fighting through the DLC, there are many combat situations in Final Fantasy 16. Leviathan’s abilities can create new tactics when facing off against all the S-Rank Hunts. Still, just because Leviathan is new does not mean you should focus entirely on it. Mix and match all the Eikon options to make Clive strong and versatile.