CR of an encounter with reinforcements

[Serum]

How might I determine how the effective CR of an encounter is modified if not all participants are present at the beginning?

For example, here is a CR 10 encounter to be used against a 5-member 6th level party, with the conditions for each creature to start participating:

• The PCs start out fighting a CR 5 creature, 5A
  
• The round on which 5A is defeated, a second CR 5 creature with ranged attacks, 5B, joins the fight at medium range
  
• one or two rounds later, a CR 7 creature with ranged attacks, 7A, joins the fight at medium range
  
• if a PC closes with 5B, a third CR 5 creature, 5C, joins the fight
  
• if a PC closes with 7A, two CR 3 creatures, 3A and 3B, join the fight

Clearly, this setup is less difficult than if the PCs started the encounter engaging all 6 creatures at the same time. Technically, since 5A dies before anyone else engages, it could be considered a completely separate encounter from the rest. If 7A and 5B spend rounds to buff themselves while the PCs are fighting 5A, how does that change the consideration?

When would you have each of these creatures roll initiative, assuming they were all aware of the PCs when the encounter began?

• Would they roll when they entered combat? In this case, they would enter combat at the beginning of the round, and then (and then at an initiative order not necessarily when they expected to be)
  
• Would they roll initiative when 5A is initially engaged, then delay (or spend actions to buff themselves) until the condition when they enter combat is triggered? In this case, by delaying, they can enter initiative order when they desire.
• Something else?

If everyone rolls initiative at once, then this would officially be considered a CR 10 encounter, even if most of the opponents are not participating in combat.

[Dasrak]

CR is a guideline, and when you step outside of the standard presumptions of the game (as you are with the staggered arrival of enemies) it becomes a less useful tool. There is no hard and fast rule to determine the difficulty of this battle. It's obviously less than a typical CR 10 and you should treat it appropriately, but that's a value judgement and nor a hard rule. In terms of XP, though, just award the PC's full XP for defeating a CR 10 encounter. Don't be a miser; sometimes XP will come a bit easier, and there's nothing wrong with that (if the PC's are leveling up too fast, move them to a slower track)

Buff spells shouldn't enter into the consideration. They are abilities that the enemies have, and are already accounted for in their CR. Having the opportunity to prebuff is just part of the tactical situation. There is precedent in which some published products do include a CR adjustment for particularly nasty ambushes, but that should be reserved for particularly exceptional circumstances.

As for when to roll initiative and when to end combat and restart initiative, that's something that you the game master should judge. Your goal should be to keep the game flowing, so make the decision that keep the kind of pace you're going for. Yes, you can pull funny initiative tricks with the delay action; try to avoid that, your goal is to keep the game flowing not promote cheesy unintuitive tactics.

[Erik Freund]

I'm only going to comment on the initiative part here:
Make sure you don't punish players for rolling high initiative. This is a common problem with reinforcements if they just show up unexpectedly in the middle of imitative rounds, as now only lower-initiative PCs can interact with them.

I would suggest this: you have predetermined conditions for determining the specific round a reinforcement will show up on. Cool. At the top of that round, before anyone goes, announce that a new challenger has entered the arena, and make them targetable. Once everyone at the table understands what he looks like and where he is, then roll the newcomer's initiative and slot them into the order.

If the reinforcer rolls high, then it's effectively an ambush, and he gets the drop on the party. On the other hand, if the reinforcement rolls more middling, then it gives the high-initiative PCs an opportunity to hamper his entry or get in some initial shots against him: as should be their right for being high initiative.

[Erik Freund]

I would also not shard off 5A as its own encounter. Implicit in the definition of an encounter is the ability to patch up wounds via slow methods, like repeated pokings with a Wand of CLW. Also implicit is for short-term buffs, like haste and displacement, to wear off.

Because of these spill-forward effects (for both good and ill), he's part of the extended fight.

FWIW, these sorts of mid-fight reinforcements are very common in 4E adventure design, and the 4E DMG encourages their use, and subsequently, has rules for building it. However, this is of little use to you, as the "CR math" over in 4E land is wildly different than in PF.

[Serum]

I'm most interested in how other GMs would feel pitting this type of APL+4 encounter against a group of PCs; I'm not looking for any concrete rules, I would have found those already unless someone talks about them in 3PP!

Dasrak wrote:

CR is a guideline, and when you step outside of the standard presumptions of the game (as you are with the staggered arrival of enemies) it becomes a less useful tool. There is no hard and fast rule to determine the difficulty of this battle. It's obviously less than a typical CR 10 and you should treat it appropriately, but that's a value judgement and nor a hard rule. In terms of XP, though, just award the PC's full XP for defeating a CR 10 encounter. Don't be a miser; sometimes XP will come a bit easier, and there's nothing wrong with that (if the PC's are leveling up too fast, move them to a slower track)

The XP gained by going through a CR 5 encounter followed by a CR 9.5 encounter is the same as the XP gained from a CR 10 encounter, so there's no worry there.

Dasrak wrote:

As for when to roll initiative and when to end combat and restart initiative, that's something that you the game master should judge. Your goal should be to keep the game flowing, so make the decision that keep the kind of pace you're going for. Yes, you can pull funny initiative tricks with the delay action; try to avoid that, your goal is to keep the game flowing not promote cheesy unintuitive tactics.

Erik Freund wrote:

Make sure you don't punish players for rolling high initiative. This is a common problem with reinforcements if they just show up unexpectedly in the middle of imitative rounds, as now only lower-initiative PCs can interact with them.

I would suggest this: you have predetermined conditions for determining the specific round a reinforcement will show up on. Cool. At the top of that round, before anyone goes, announce that a new challenger has entered the arena, and make them targetable. Once everyone at the table understands what he looks like and where he is, then roll the newcomer's initiative and slot them into the order.

If the reinforcer rolls high, then it's effectively an ambush, and he gets the drop on the party. On the other hand, if the reinforcement rolls more middling, then it gives the high-initiative PCs an opportunity to hamper his entry or get in some initial shots against him: as should be their right for being high initiative.

Agreed. While I was writing the second option, I realized that it seemed somewhat unintuitive, barring mitigating circumstances. These circumstances would be something like the creatures using stealth keep from being detected, opposed by PC perception checks.