Meliodas form isn t magic or a demon power. It steals meliodas of his emotions and makes him temporarily heartless the way he was before betraying the demon clan.
Meliodas is a performance orientated workout that prepares the body for combat moves.
Meliodas demon form full body. Meliodas assault mode is a form which he achieves when he unleashes his demonic powers to the fullest extent. Full demon form when meliodas unleashes his full demon form he accesses the powers that he possessed once in his prime as the demon king s successor. This mode is first revealed when he desperately uses his full power and breaks free from melascula s cocoon of darkness antan no mayu an attack which traps their opponent in a pitch black sphere enveloped in darkness.
Although meliodas managed to slowly control it as the series was progressing. Assault mode this state is reached when meliodas unleashes his demon powers achieving immense powers. But this comes at a cost.
When he access his demonic heritage part of meliodas body is covered in shifting black matter which he can manipulate and shape as well as extend it outside his body. While using it he is bloodlusted and looses the capacity for rational thought. This form also allows meliodas to access his full power.
Meliodas is half demon and can access his demonic heritage to give himself a large boost in physical capacity as well as other abilities. So natsu can t turn into a full dragon. After absorbing all the commandments and becoming a vessel for the demon king meliodas body became that of a tall and muscular adult with long hair a white full body leotard golden boots and gauntlets and two long clawlike protrusions coming out of a dark matter accumulated on his back.
As such it focuses on tendons and satellite muscle groups to produce coordinated strength and power which is what actually turn a body from a mass of bones muscles and sinew into a dangerous weapon. As he gets angrier the black mark spreads until it covers more and more of his body his power increasing as the mark spreads. When meliodas gets angry enough he can tap into his demonic powers forming a black mark on his forehead and giving him a boost to his strength speed and durability.
The worst that happens is dragon scales coming out of his body when he uses dragon force but that s it. According to melascula back in the days when meliodas was in command of the ten commandments even they were in fear of him while in this mode. He can use this to block attacks on his body to form giant claws and blades to attack or to fly by forming a wing like structure around his arm.