Minecraft wiki entry for Evoker, associated text: Fangs appear no lower than the feet of the lowest combatant and no higher than one block above the feet of the highest combatant. Fangs attempt to appear on the highest opaque block between those two extremes, but fail to spawn if they are obstructed by a solid block. In practice, this means that fangs cannot spawn inside deep pits or on top of high walls, but may, for example, go up a staircase if the target is at the top and the evoker at the bottom, or vice versa. Likewise, an evoker floating in a boat on water cannot summon fangs against a player swimming or floating in a boat because there are no opaque blocks from which the fangs can appear.