Marco ate the Tori Tori no Mi, Model: Phoenix, a Mythical Zoan-type of Devil Fruit which allows him to transform into a phoenix. It is an immensely powerful type of Zoan Devil Fruit, said to be the rarest type, even rarer than Logia fruits. It is because of this ability that he is identified as “Marco the Phoenix”.
While this fruit’s power is classified as a Zoan, due to the uniqueness of the phoenix, the additional powers it grants may be considered similar to those of Paramecia (the somewhat “superhuman” power that affects the user’s body: regeneration) and Logia (the generation of blue phoenix flames).
This Devil Fruit gives Marco increased physical attributes (as is with all Zoan types) and the rare ability of flight, as well as the possibility of launching aerial attacks. During the Battle of Marineford, Marco showcased his aerial maneuverability by traversing quickly across the battlefield and supervised the assault in Whitebeard’s place. He can transform his feet into phoenix talons, with which he can slash his enemies.
The fruit’s greatest attribute, however, is that by transforming into a phoenix of blue flames at will, Marco can regenerate any wounds with the blue fire he generates, much like how a phoenix is said to rise from its own ashes. Because of this, Marco is very resilient and can take a huge amount of damage in his phoenix form with no visible consequence other than the time taken to regenerate. These blue flames are not hot and they do not burn or spread like normal fire; instead, they are what allow him to heal, though there is a limit to the regeneration and are referred to as the “Blue Flames of Resurrection”. Marco can recover from any damage he accumulates in his human form by reverting to his phoenix, or hybrid phoenix form, or just by summoning blue flames around his body.
These blue flames are referred to also as the “Flames of Restoration” in fact the healing properties of Marco’s blue flames can also be used to treat other beings, albeit to a more limited extent, allowing Marco to act as an efficient doctor.
Marco’s control over his transformations seems to be the greatest of any other Zoan user shown so far, as unlike most other Zoan users (Laffitte, Chopper, and Onigumo, possibly being the only known exceptions), Marco can transform selective parts of his body into phoenix form without changing his whole appearance. In his most commonly seen human-beast form, Marco only transforms his arms into wings of fire (sometimes choosing to maintain his hands) so that he can fly and still attack with standard kicks.