Whatever Wednesdays: Nested monsters
So, I'm starting a new thing this month, it's called "Whatever Wednesdays" Where I cut away from the month's theme so I can bring up any old topic that Is on my mind.
This time I want to talk about what I like to call "Nested monsters". I can't take credit for this idea, I've seen it in a very few 3.0 and 3.5 monsters, but more recently I've seen it used very well by one of my closest friends and sometimes DM's. Yes, even I get to be a player sometimes.
The idea is simple. Take a monster and put another monster inside of it. When the first monster is killed, a second one emerges. A magma elemental is shattered and out comes an earth and a fire elemental. A zombie is felled only to have its chest explode in bats. The skeleton suspended inside a gelatinous cube is actually an animated skeleton. You get the idea.
When creating this type of encounter the first thing to think about is what makes it different from fighting a pair of enemies. Firstly, it's different because it's not expected. the first few times this happens it might be a complete surprise. The party thinks they've won and then suddenly a monster explodes onto the scene, attacking them just when they thought they could relax. The other main benefit is that it allows the players to focus on one foe at a time, but still deal with the same number of creatures. This makes the overall encounter easier than facing two foes at once. You can also use this to fake out your party. A wolf explodes into a hellwasp swarm and suddenly staying grouped together and using melee weapons becomes a far less viable tactic.
Naturally an encounter like this needs to have some adjustment to the CR, but it's a very easy one to make. Having one creature inside another raises the cr of the encounter by 1 if both creatures have the same CR. so a cr 4 monster that explodes into a CR 4 monster when slain is a CR 5 encounter. From there all the regular adjustments apply. If there are two such nested monsters of that power the total CR would by 7. We can also then create a mixed nested monster. a CR 4 monster might instead explode into 2 cr 2 monsters, or a CR 3 and a CR 1, and so forth. And it goes without saying to always base things off the strongest monster, regardless of if that's the monster inside or outside.
This same formula can also be used for multi stage boss fights. An ogre beaten down and then overcome with a sudden surge of ferocious power, selling its soul on the spot and becoming a half demon might heal entirely and gain new powers, essentially becoming a whole new enemy, and as such this same formula would be appropriate. Big sweeping multi stage fights are a hallmark of great videogames, and exist in many forms through movies and literature and I think it's only fair Pathfinder get a shot at it too.
Try this system out and see where it takes you, hopefully it can lead to some great encounters.