The illusioner is an unused illager armed with a bow. It attacks using illusions and can also fire arrows.

Behavior Although they do not naturally spawn in raids, illusioners spawned by the player attack players, adult villagers, iron golems and wandering traders within a 16416 cubic area.

An illusioner joins a nearby raid if it occurs or joins a patrol if sufficiently near a patrol captain.

The illusioner attacks with its spells and its bow, firing an arrow every second. Unless the illusioner is summoned using commands, it cannot attack with a non-bow item in its hand, cannot use tipped arrows in its offhand slot (but can use spectral arrows) and cannot use enchanted bows. Also, it is the only illager that can see its target through walls.

The Illusioner moves quickly on a semi-circular fashion and always tries to maintain a consistent distance between itself and the player, strafing from left to right and vice-versa, retreating if the player gets too close or advancing if the player retreats.

Finally, the illusioner has two spells: a spell that blinds its opponent and a spell that summons visual pseudo-duplicates and makes the illusioner invisible.

Spawning information An illusioner has no spawn egg in the Creative mode inventory. An illusioner can be spawned only using data packs or the /summon illusioner in-game command. Illusioners do not spawn in woodland mansions or pillager outposts, nor at raids or patrols. Similarly to the other unused giant and zombie horse mobs, illusioners do not spawn in any biome, monster spawner or world-generated structure.

More illusioner facts here : https://minecraft.fandom.com/wiki/Illusioner