The month is coming to a close but before it does let's finish up looking at schools of magic with an encounter based on transmutation.
CR 5- Summon monster 5 trap- 1,600
Basic premise: A transmutation themed Puzzle with dangerous consequences and multiple ways of Solving it. As the players enter the room a huge clay jar shatters. By calling and defeating elementals in the right order the jar can be reformed.
The breakdown: As the players enter the camberthey hear the tremendous sound of a 10 ft clay pot shattering and a stone door sliding shut. A voice speaks out "That which was unmade, must be remade before our eyes". The room is smooth stone with four large obvious engravings equally spaced through ought the room and each with a large eye engraving above it, all facing where the pot once stood. One button has a mountain, one with waves like water, one with the shape of a flame, and one with lines like air. When a button is pressed a Large sized elemental will appear corresponding to the button (earth for rock, air for air ect). As elementals are defeated in the correct order the pieces of the pot will change. When an earth elemental is defeated the pieces will crumble into dry clay. If a water elemental is defeated after that the clay will soften. If an air elemental is beaten next the pot will form into a proper pot shape and begin to dry. If at this point a fire elemental is slain the pot will shrink slightly and harden back into proper shape and the door will open. Defeating an elemental out of that order ruins the currently gained progress.
There are several ways the players can make this challenge easier on themselves. A dc 13 profession or craft check related to pottery tells the players what the order should be. And if the players have the time and resources they could even try to repair the pot. It has 100 hp when fully repaired and so any spells such as Mending or the like will have to undo that much damage.
A clever player can also create an illusion of the pot and that will trick the door into opening, as will any other object of the same exact shape and size, regardless of the material its composed of. Further the elementals always appear in the nearest space in front of their button, meaning the players can ready to attack them as they appear.
The Disable device DC to bypass this puzzle is 25, and a failed attempt calls another elemental, always of the incorrect type to complete the puzzle. A summoned elemental remains for 10 rounds, and if not slain within that time it vanishes and does not count towards the puzzle. Only the order they are destroyed in matters for the purpose of solving the puzzle.
Tale of the tape: This is essentially a CR 5 summon monster trap with an automatic reset. Normally the difficulty would be higher, since the potential exists for multiple foes to be fought at once, but this is offset by the fact that the players can defeat the trap without a single battle, and that after the first elemental they should be ready for more.
Variations: If your campaign has a different elemental focus you can feel free to change the symbols or even types of elementals to match, though you'll likely need to rethink the object being created. You can also scale the CR up or down by changing the strength of elemental being summoned. You should base the CR on the elemental, NOT the normal cr for that type of trap. So a trap summoning CR 11 elder elementals should be CR 11, Despite mimicking a level 8 spell.