Evokers are enemies that only spawn in woodland mansions, which are found in the roofed forest biome. They look like villagers but use magic to attack you (not to be confused with vindicators, which hold an iron axe). They drop the Totem of Undying, which is used for the Cheating Death achievement.
If you want to do this on a fresh seed as opposed to uploading an Xbox One save, use the seed -518068014 (v. 1.2.0). Walk about 100 blocks west of the spawn to find a large woodland mansion. I found an evoker on the second floor.