Leviathan-Leviathan Fruit: This Paramecia-type Devil Fruit grants the user the power to manipulate and control leviathans, colossal sea creatures of immense size and power. The user doesn't transform into a leviathan, but rather gains a psychic link and control over them.

Abilities:

• Summoning: The user can summon leviathans from the ocean, their size and number depending on the user's strength and control. Smaller leviathans might be summoned for quick attacks, while larger ones could be used for devastating blows or as living fortresses.

• Leviathan Manipulation: The user can control the summoned leviathans' movements, attacks, and even their biological functions to a certain extent. This includes directing their attacks, using them for transportation, or even using their bodies as shields.

• Oceanic Awareness: The user gains a heightened sense of the ocean's currents, the location of leviathans, and other underwater phenomena. This allows for strategic planning and evasion.

• Leviathan Enhancement: The user can potentially enhance the leviathans' strength, speed, and durability through their connection, making them even more powerful.

• Defensive Capabilities: The leviathans can act as formidable shields, protecting the user from attacks. Their immense size and strength make them difficult to overcome.

• Offensive Capabilities: The leviathans' attacks are devastating. They can use their immense size to crush opponents, their powerful jaws to bite, and their tails to whip with incredible force. They can also create powerful tidal waves.

• Symbiotic Relationship: Over time, the user might develop a symbiotic relationship with their leviathans, understanding their behaviors and communicating with them telepathically.

Weaknesses:

• Seastone Vulnerability: Like all Devil Fruit users, the user is vulnerable to Seastone, which weakens their abilities and control over the leviathans.

• Limited Control: The user's control over the leviathans is limited by their own strength and willpower. Overextending themselves could lead to a loss of control.

• Dependence on the Ocean: The user's abilities are heavily reliant on the presence of water, making them significantly weaker on land.

• Leviathan Exhaustion: Summoning and controlling multiple large leviathans can be incredibly draining, leaving the user exhausted.

• Counter-Attacks: Powerful attacks that can harm or kill the leviathans will directly impact the user's abilities.

• Environmental Factors: Severe storms or other oceanic disturbances can interfere with the user's control over the leviathans.

Potential Combinations: The Leviathan-Leviathan Fruit could be combined with other Devil Fruits to create even more powerful abilities. For example, a combination with a Logia-type fruit granting control over water could significantly enhance the user's control and power over the leviathans and the ocean itself. A Paramecia-type fruit granting enhanced strength or physical abilities would also greatly amplify the user's capabilities.