Alastor´s heinous prank

You twist and reshape a spell just as a creature is casting it. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect, the twisted spell takes the form of a wild elemental that is summoned next to the target, the wild elemental takes the form of any beast with a CR equal to the level of the spell interrupted. The wild elemental is an elemental and is not considered a beast, the wild elemental has immunity to the type of damage the twisted spell causes if the spell can cause harm. Its alignment is chaotic neutral and is hostile to all creatures present during its summoning, Its objective is to cause as much havoc and collateral damage as possible.
At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the interrupted spell has no effect if its level is less than or equal to the level of the spell slot you used.