The Phoenix Form is a powerful transformation granted by the Tori Tori no Mi, Model: Phoenix, a mythical zoan-type devil fruit consumed by Marco, formerly the Whitebeard Pirates' 1st Division Commander. This gives Marco the ability to transform into a phoenix or a phoenix-human hybrid at any time, earning him the nickname "Marco the Phoenix."
Well, Marco's full-phoenix form is that of a large blue-flamed bird with long feathers on the tail and wing-like arms. Such a form is often represented in the anime with bright blue and yellow flames, which evokes a rather impressive view to emphasize the mythical tinge of his powers. The phoenix transformation hugely enhances Marco's body, making him stronger, faster, and more resilient.
One of the most striking features about Marco's Phoenix form is his regenerative powers. He can just cover the part of his body which is wounded in his phoenix flames to instantly heal any injury in this form. This was notably seen during the Marineford War when Marco repeatedly recovered from attacks that would have been lethal to many other characters.
His phoenix form also grants Marco the ability of flight, which aids him in his general fighting capability by addition of aerial battle and easy movement over large distances. This is often clothed with a display of blue flames as he flies in the anime, adding flair to the presentation of his powers. This has been useful in so many battles in which he is able to support from advantageous positions or attack from above.
In this transformation, Marco takes on a hybrid form between that of a human and a phoenix. That is to say, Marco comes into his human form but can manifest the characteristic features of a phoenix, such as flaming wings or sharp talons on the legs. Such a hybrid form has the ability to mix human agility with phoenix powers in a very versatile way and hence proves quite useful in close combat.
This does not limit Marco's Phoenix powers to being only an offensive tool. Using flames is possible for him to heal at a much slower pace compared to healing himself. He performed the healing once during the Wano Land arc, where Marco used his powers to treat victims of the Ice Oni virus.
The combat techniques are unique to him in his Phoenix form. He may use his wings to block an attack with his regenerative flames, which would negate the damage. Another technique includes the use of talons to rend and slash with great power, often augmented by the momentum of his flight.
It is also important to note that Marco's power in his Logia does not stop at the simple aspect of full-body change. He can transform parts of his body partially; he can turn only his arms into wings. This shows how truly experienced and powerful he is, with powers bestowed by the Devil Fruit.
Even in Phoenix form, Marco is not invincible. His regenerative abilities are very potent but not limitless and can thus be exhausted with continuous or remarkably strong attacks. Furthermore, like all users of the Devil Fruit, sea water and sea stone can render Marco's powers null and helpless.