Java Edition:Illusioner
Illusioners were first added to the game in 17w16a (a 1.12 snapshot), but were never made encounterable in survival mode. However, a few April Fools' snapshots allowed for them to spawn. Illusioners can also be used to obtain the illusioner killed statistic and the killed by illusioner statistic.
Obtaining
20w14∞ Dimensions
One of the random mobs that 20w14∞'s random dimensions can spawn is the illusioner. An example of a dimension with this property is the "a" dimension. Once found, they must be transported to another dimension that will remain after updating to later versions. The illusioner will be hostile towards the player, making this difficult, but the illusioner can be moved through a nether portal back to the overworld, or through an end portal using end portal frame items. Illusioners rarely (6% of the time) are raid captains, and have ominous banners on top of their heads.
23w13a_or_b's natural_spawn_replacement Vote
In the April Fools' snapshot 23w13a_or_b, the vote natural_spawn_replacement can be used to replace the natural spawning of any mob with any other mob in the game, including illusioners. Once approved, all mobs as specified by the vote, when spawning from natural spawns, will be replaced with a mob as specified by the vote.