Advancing monsters - do the CR rules always hold?

[Kalendraf]

Last night, I worked on an encounter for an upcoming adventure. I decided to advance a certain monster (large magical beast) which is normally CR5 with 6HD. (I'm staying rather vague in case one of my players swings by). According to the guidelines, it raises 1 CR for every 3HD, and another CR if it increases size category. I decided to try bumping it all the way to 18 HD which also increases the size to huge. By the formulae, the 12 extra hit dice should raise it by 4 CR-levels, and the new huge size will bump it 1 more CR. Thus, the result is theoretically a CR 10 monster.

However, as I began calculating all the other numbers for things like atk, damage, hit points and so forth, it began to look like this particular creature is probably stronger than most other CR10's. It has amazingly high attack rolls (18 bab with magic beast tracking like fighter progression) which combined with its high strength means it basically doesn't miss. It gets multiattacks, but the damage seems reasonable (2d6 + 9) for a CR10 monster. It's got over 200 HP. The quickest thing I thought of comparing it to was a dragon, and the ones I looked at were CR11, but I suspect in a battle, this advanced magical beast might tear up a party worse than the dragon. Though it lacks a breath weapon like a dragon, but it has some other special attacks instead which can be similarly damaging.

I'm attempting to balance this encounter for the party. If indeed it's only a CR10, then I may mix in some normal-sized versions of the same beastie, making the encounter somewhat more challenging, but not overly deadly for the party. My hunch is that this particular magic beast is closer to a CR11 or even CR12, even though the formula only computes it as a CR10. Thus, having it encountered alone may be enough to properly challenge the party. Do I trust the formula? Or are there known problems with advancing monsters I need to be aware of.

 

[Thanee]

Well, given that CR doesn't even hold for non-advanced creatures, the answer to that question can only be no.

CR is a guideline, nothing else. Sure, the rules are based on CR, EL, XP, etc, but they are not accurate enough.

Also, the individual party strength is not figured in at all. One 10th level party can easily be like twice as strong in combat as another 10th level party (4 PCs each). So, as the DM you have to make some judgement calls there.

Trust yourself, not the formulas.

Bye
Thanee

 

[Kalendraf]

That's the way I was leaning. I realize that CR's are just guidelines, but I haven't done a lot of monster advancements of this type. Most of time it's just been a minor bump of 1 to 4 HD or adding class levels to a humanoid which seems more straight-forward to me.

 

[Methos of Aundair]

Another thing, as I recall it states under advancing creatures that if the creature adjusts in size due to racial advancement it does not gain the +1 CR due to a larger size.

I use advanced creatures every now and then and I don't find the CR differences too bad. If the creature seems too powerful for the new CR or too weak, I merely modify the CR after the battle. Though remember, I have found a lot of times players go into the battle seeing a spectre (thinking its a normal 7HD) and fight it as normal, and not realizing it until its too late that it is advanced (14 HD). It adds to the fun. Too many players see creatures and know their stats from memory, so advanced creatures is another way to surprise the party.

 

[Scion]

How does your guy look vs the Gargantuan monstrous scorpion? or the colossal one?

Edit: didnt get to finish my post. Creatures with incredibly glaring weaknesses seem to take a middle ground. Taken in their element they are way above the CR they are rated, taken outside of it and they are way below. The scorpion is a good example of this. No flight means that vs opponents that fly he is a push over, vs people who try to take him headon and it looks like a tpk possible situation.

so it may very well be that the CR is right on for the way the system is set up. But depending on how your group handles it changes what effect it has on them greatly.

 

[dcollins]

Do I trust the formula?

No. That in particular is one of the least reliable parts of the CR system. (In general I think the system is pretty useful, I don't begrudge the 3rd Ed. designers falling down on one part of it.)

Skip Williams had an article called "How to Create a Monster" that had thumbnail CR guesstimation that I personally use in place of that CR advancement rule. More or less I do this: a monster's CR is average hp/15 + X. Figure out what X is for the original monster. Then calculate using the same X for the advanced monster. Rough, but better than the book rule for me.

 

[Kalendraf]

Thanks for the responses. The encounter went fine tonight and turned out to be balanced for my party (4@10th, 1@9th, 1@8th). In case you're wondering was the creature was, I advanced the Manticore all the way to 18HD, which I then called MOAM (see below) There was a lead-in encounter featuring 5 regular manticores which the party managed to defeat, killing 2 and forcing 3 to flee. One of them "ran to mommy" and that brought her out to play. As a result of all this, the party began the battle buffed somewhat, and got an extra round to react before she could get into range.

The battle only lasted a couple rounds. She got off 2 major volleys of spikes, managing to drop the previously wounded bard to negative hitpoints care of 2 spikes, one scoring a crit. The rogue was in bad shape as well due to a similar pair of hit & crit spikes. However, the rest of the party beat her up pretty well, dishing out something like 150 damage via ranged attacks and spells. She could have easily killed both the bard and rogue with another volley, but its likely the remaining characters would have killed her in return, so she offered a truce which the party grudgingly accepted. She promised not to kill them, so long as they left her hunting grounds.

Here's my updates:

M.O.A.M. (MOTHER OF ALL MANTICORES)

Huge Magical Beast
Hit Dice: 18d10+126 (225 hp)
Initiative: +6
Speed: 30 ft. (6 squares), fly 50 ft. (clumsy)
Armor Class: 19 (–2 size, +2 Dex, +9 natural), touch 10, flat-footed 17
Base Attack/Grapple: +18/+35
Attack: Claw +27 melee (2d6+9) or 6 spikes +21 ranged (2d6+4/17–20)
Full Attack: 2 claws +27 melee (2d6+9) and bite +25 melee (2d6+4); or 6 spikes +21 ranged (2d6+4/17–20)
Space/Reach: 15 ft./10 ft.
Special Attacks: Spikes
Special Qualities: Darkvision 60 ft., low-light vision, scent
Saves: Fort +18, Ref +13, Will +7
Abilities: Str 28, Dex 14, Con 24, Int 8, Wis 12, Cha 9
Skills: Listen +11, Spot +15, Survival +1
Feats: Flyby Attack, Multiattack, TrackB, Weapon Focus (spikes), Improved Initiative, Hover, Improved Critical (spikes), Wingover
Environment: Warm marshes, Hills
Organization: Solitary, sometimes encountered with younger, normal manticores
Challenge Rating: 10
Treasure: Standard
Alignment: Usually lawful evil
Advancement: 18 HD (Huge)
Level Adjustment: +3 (cohort)
A Mother of All Manticores (M.O.A.M.) is about 15 feet long and weighs about 3500 pounds. The name is slightly misleading, as it may refer to either a male or female of the species. The acronym for Father of All Manticores (F.O.A.M.) is not used as it would seem to imply a fun, squeezeable creature which is directly opposite of what it actually is. In fact, a M.O.A.M delights in squeezing its victims to death. M.O.A.M.s speak Common.
COMBAT
A M.O.A.M. begins most attacks with a volley of spikes, then closes. In the outdoors, it often uses its powerful wings to stay aloft during battle.
Spikes (Ex): With a snap of its tail, a manticore can loose a volley of six spikes as a standard action (make an attack roll for each spike). This attack has a range of 180 feet with no range increment. All targets must be within 30 feet of each other. The creature can launch only twenty-four spikes in any 24-hour period.
Skills: *M.O.A.M.s have a +4 racial bonus on Spot checks.