When the series started, Mukuro was the first person who showed the power of illusions. An illusionist uses their psychic powers to make their enemy feel, see, hear, or smell things that aren’t there. Basically, every illusion is dealt through the mind. In the case of a first-class Illusionist, it's possible to even kill the enemy with illusions; for example, Chrome created an illusion of lava erupting and Viper (Mammon) said that if he had believed it was real for one second, he would have burned. This proves, however, that if the enemy is aware that they are experiencing an illusion, it will have no effect on them and does not exist. Normal illusions take control of fear and forces other people to be afraid.
When an Illusionist use their illusions combined with the power of Mist Flames, they will create Real illusions and the illusion would be experienced in reality.
During the introduction of Mukuro's First Realm, it was stated that it grants him the power to create Real illusions capable of manifesting in the physical/real world. However, it wasn't until during the Future Arc that Real illusions were seen. Sometimes real illusions are hard to tell between normal illusions. The real illusions also have the power to hurt people.
Mukuro also made use of this to materialize himself during the Mist Ring Battle to fight Mammon, when fighting Glo Xinia in Kokuyo Land with Chrome in the future with Ken and Chikusa, and during the Choice Arc, when Tsuna and his friends were fleeing from Byakuran.
When two illusionists are going to fight, the role of an illusion is different. An illusionist will try to enchant his enemy with his illusion (this will mean controlling the enemy’s brain which governs their five senses). The enemy needs to survive by completely recovering this control and casting an illusion that will mirror the other illusion. The first one that has his illusion completely cast/returned by the opponent and be unable to recover their control completely will lose, and then they will be left to their opponent's mercy. For example, Chrome created an illusion of lava erupting, but Mammon wasn’t completely fooled and casted an illusion that the fire/lava would be turned into ice. Chrome, unable to recover, was turned into ice and slammed against the wall by Mammon as a result.
Illusionists, or better known as spellcasters or technique-users, are the ones that cast illusions or psychic powers into the enemies' mind and thus, controlling their enemies' brain which governs their five senses, making them lose their perception and senses with things that aren't even there. Illusionists are weak to physical pain.
In conversation during chapter 336, it was hinted that illusionist have abilities to possess people; when Mukuro's body was possessed by Daemon, Reborn said that he for the first time heard of "an illusionist to have their body stolen by another illusionist while possessing someone." However, so far, the only people capable of performing this feat were Mukuro and Daemon. This might be because it needs specific conditions necessary for it to work and even without any specific conditions, it's still an exceptionally difficult technique.
Also, apparently, it seems that if an illusionist cus off their attachment with his/her own body, the illusionist will become a living spirit with his/her illusions becoming stronger from time to time. Daemon Spade is, for now, the only one who is known to have done this for years. But to do this means that the illusionist must have a body to possess or else its spirit form will disappear and the illusionist will finally die. How much power the illusionist would gain is dependent on the vessel they are possessing. According to Daemon, this action was considered taboo.
- Normal Rings: Normal Rings can produce a Mist Flame to construct the Illusion into a real Illusion. The higher the quality of the Ring, the more power an Illusion will have.
- Hell Rings: Hell Rings are Rings that can create an Illusion more powerful than an Illusion with a normal Ring. Hell Rings can also consume the Illusionist.
- Normal Illusions are illusions that the Illusionist puts in the mind of one person, while Real Illusions are illusions that which are put on reality and have an actual existence and mass.
- The Mist and Desert Flames, the Illusions Flames of the Dying Will Flames of the Earth and Sky, are said by Daemon to be the "joker" Flames of Illusions.