Level Up (A5E) Calculating Encounter CR for Monsters Arriving Late

[FuzzyBunny]

I'm working on The Final Battle in my campaign, and trying to get it to the right level of difficulty. Is there a factor (x0.5, x0.75, etc) to be applied to monsters who came late into a battle? The D&D action economy favors lots of actions/monsters. Additionally, damage earlier in a fight is worth more than damage later (since damage that hits earlier has the chance to reduce a combatant to 0, rendering their damage per round at 0). I'm trying to understand this in order to make my final battle truly epic and challenging, without making it impossible.

Note that I play O5e, but I am posting under A5e because I am a huge fan of the Monstrous Menagerie and its rigorous CR calculations.

 

[Stalker0]

Probably the easiest way to do this:

Normally when you have monsters that have variable damage effects round to round (such as a dragon's breath weapon on round 1, claw/bite on rounds 2-3, etc), you average 3 rounds of damage.

So you could just assume 0 for the 1st round, then average in rounds 2 and 3, which will give you your new average damage (and then update teh monster CR appropriately).

Not perfect by any means but it should give you a ballpark.

 

[FuzzyBunny]

That's a good idea. It leads me to two questions:

1) Is there a guideline on updating monster CR based on damage alone? Since a monster gains ~5 damage/round and 15 HP for each level its CR increases, my assumption is 15 HP is worth 5 damage. So if a monster's damage goes up by 10, and no other statistics change, that's roughly equivalent to an increase of 1 in CR.
2) I am hoping this is a 5 round battle, maybe a little more, since it's the final battle. Your calculations make sense, but how would I factor that in? Monsters would arrive probably round 3.

 

[lichmaster]

1) Is there a guideline on updating monster CR based on damage alone? Since a monster gains ~5 damage/round and 15 HP for each level its CR increases, my assumption is 15 HP is worth 5 damage. So if a monster's damage goes up by 10, and no other statistics change, that's roughly equivalent to an increase of 1 in CR.

In LU you can increase/decrease a monster's HP by 10% without altering its CR. Beyond that, you should reduce or raise its AC by 1, or its damage per round by 5%, for every 5% of hit points you varied from the base hit points.

Your case would be the opposite: since you already increased its DPR by X%, you need to figure out by how much % you should decrease its HP if you wish to keep the CR fixed. Otherwise you increase CR it by 1 (+5 dmg, +15 HP), and see if the difference in HP is small enough that no mod to damage is necessary.

2) I am hoping this is a 5 round battle, maybe a little more, since it's the final battle. Your calculations make sense, but how would I factor that in? Monsters would arrive probably round 3.

You could do this in different ways. One way could be to conceptually split this encounter in two, one easy and the other hard. The easy encounter will probably last less than 3 rounds, while the other more likely 3 or more, so the "sum" should be slightly more than 5 rounds.

That said, my favorite way of controlling the length of a fight is by doing it directly, by either having more monsters arriving/some of them healing if needed, or having them scatter/die quicker if I want to cut it short. Or you could add complications on either or both sides (environmental hazards, another threat joining the fray, etc)

 

[FuzzyBunny]

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[FuzzyBunny]

In LU you can increase/decrease a monster's HP by 10% without altering its CR. Beyond that, you should reduce or raise its AC by 1, or its damage per round by 5%, for every 5% of hit points you varied from the base hit points.

Your case would be the opposite: since you already increased its DPR by X%, you need to figure out by how much % you should decrease its HP if you wish to keep the CR fixed. Otherwise you increase CR it by 1 (+5 dmg, +15 HP), and see if the difference in HP is small enough that no mod to damage is necessary.

I am not following. I may need to clarify my question. What I'm asking is: if I wanted to increase a monster's CR by 1, but only by increasing damage and not HP, could I do that by increasing DPR by 10? Since increasing a monster's CR means increasing its DPR by 5 and HP by 15, I assumed 5 DPR is equivalent in value to 15 HP. Therefore, I could increase a monster's CR by 1 by increasing it's damage by 5 + 5 (exchanged for 15 HP) = 10. This is a rule of thumb I've been playing with.

You could do this in different ways. One way could be to conceptually split this encounter in two, one easy and the other hard. The easy encounter will probably last less than 3 rounds, while the other more likely 3 or more, so the "sum" should be slightly more than 5 rounds.

A little background: I'm going to run an Angry GM-style Paragon Monster, with Paragon Fury. He will start out as a CR 10 monster with one turn, and once he is killed, transform into a different CR 10 monster with 2 turns (which is in accordance with this type of monster). So the first half of the battle will definitely be easier, and the latter will be hard. But since the players won't have time to rest or even drink a potion between battles, I don't think it makes sense to calculate CR as two separate battles.

That said, my favorite way of controlling the length of a fight is by doing it directly, by either having more monsters arriving/some of them healing if needed, or having them scatter/die quicker if I want to cut it short. Or you could add complications on either or both sides (environmental hazards, another threat joining the fray, etc)

Definitely good ideas. But adding minions is about flavor and giving my O5e Battlemaster Fighter something to do while the flying dracolich spellcaster stays at range and has no reason to be grounded. I just don't want to go overboard and make a battle that's too difficult.