This encounter, which I named after the Neil Finn song, is very flexible depending on the level of your campaign and the materials you have available. The general gist of the encounter is as follows; feel free to adjust it based on how your game works. This is most likely a boss fight in a dungeon.
The party is in a dungeon and enters a room where there are wintry conditions – ice on the floor, wind, maybe a light flurry falling, etc. However, things aren’t going to stay this way. The room is enchanted and the weather conditions will change quickly. While the first round of combat in the room is going to take place in the winter, at the beginning of every combat round, roll 1d4 and the season will change, as follows:
1 = winter
2 = spring
3 = summer
4 = fall
Information about weather effects is in the Core Rulebook, in the “Environment” section. Each season has the following conditions:
1. Winter: Much of the ground is covered in ice. It costs 5’ extra of movement to enter a square covered in ice; the DC of Acrobatics checks performed on ice increases by 5, and to run or charge successfully over a square that is icy requires a DC 10 Acrobatics check. Determine the penalty for failure however you like; I would suggest either halting the player’s movement in that square or having the player fall prone.
2. Spring: It is foggy, raining, and windy. The rain is token, but puts out normal fires/torches. The wind is “strong,” as described in the CRB. It causes a -2 penalty to ranged attacks. The small fog bank limits visibility to 5’, grants 20% concealment to creatures at that distance and total concealment to anything farther away (50% miss chance).
3. Summer: It is extremely hot, and the battlefield remembers a desert. There is scrubby underbrush covering part of the battlefield, which costs 5’ extra of movement per square.
4. Fall: Several fallen trees crisscross the battlefield, providing varying degrees of cover. If a character is located in a square where there would be a fallen tree, the character is knocked prone in an adjacent square (GM’s decision). A thin carpet of leaves covers at least part of the battlefield, conferring a -4 penalty on Stealth checks (crunch crunch crunch…)
Depending on what you have available, you may need to plan the size of your room in advance. If you have a standard Chessex battlemat, which is about 24×24 squares, you can divide it into 4 12×12 square rooms. When the weather change occurs at the top of the round, move the players around to the same spot in the appropriate season’s room. Draw the difficult terrain and cover in advance. You may wish to cover the other seasons with opaque paper until the weather change mechanic is revealed. If you are using a system like Roll20.net, you can make separate areas for each season, each the same size, and move the players around.
What you choose to put in this room will change based on your party’s composition and level, but I chose a powerful druid. The fey friend model from the NPC Codex (http://paizo.com/pathfinderRPG/prd/npcCodex/core/druid.html) might be a good choice for CR 13. To further spice up the encounter, remove the druid’s domain powers and introduce an animal companion.
Treasure will, of course, vary based on the monster.
Treasure varies depending on CR.