Last night in our 3.5E campaign, we fought an entomber, from Libris Mortis, the Book of Bad Latin (also undead). The entomber’s primary mechanic is, shock of shocks, entombing. Every time it connects with an attack, the target has to make a non-trivial Reflex save or become buried in a shallow grave. Escape requires the assistance of allies (which requires two turns and provokes attacks of opportunity) or two successful grapple checks. It’s actually a pretty neat monster with a pretty neat mechanic, and its Challenge Rating was only two higher than the party level, which makes it a difficult encounter where a character might go unconscious but where nobody should die.
The problem is specifically how the numbers worked in the fight. The creature hit the party’s defender on a 13 and the party’s striker on an 8, which is pretty reasonable. Its damage could knock out a player in three hits, or two hits if the monster rolled well for damage, which is fine for a brute. But the Reflex save against its entomb ability is fairly difficult, and the grapple check to escape was worse. The party rogue could avoid being entombed by rolling a 9 or higher, but needed a natural 20 to escape the grapple (which, recall, had to be done twice). The barbarian could escape the grapple on a mere 14. So with one hit, the monster gave a player a 50% chance to not participate in the encounter for a minimum of two rounds, most likely five rounds or more. We all got knocked underground, lost the combat, and the cleric had to dig everybody out and heal them before the monster came back to finish us off.
I think it’s important that every DM realize that monster levels and Challenge Ratings are a suggestion rather than a hard and fast rule. Monster CRs are designed for a theoretical perfect party of a healer, a defender, a damage-dealer, and a caster (specifically, Jozan the cleric, Tordek the fighter, Lidda the rogue, and Mialee the wizard). Any variance in this party creates a different situation that can lead to a different combat. Switching the rogue to a barbarian makes it easier to fight undead but makes the party more susceptible to spells that target Reflex, switching the cleric for a druid increases versatility at the expense of some healing, and so forth.
Consider the entombed. For the playtest party, Jozan could turn this undead creature on a difficult but possible roll of 19 on 1d20+Cha. Mialee can use single-target damage spells to hurt it or control spells to limit its reach. Lidda is largely irrelevant, but she can spend turns digging out entombed allies and is very likely to make her Reflex save. Tordek is pretty much unchanged, unless he has a silver weapon, which is unlikely but incredibly handy.
Now consider our party of three melee characters: a cleric, a barbarian, and a rogue(-like thing). We had nobody who could stay away from melee combat against the terrifying melee specialist. It was impossible for our cleric to turn it. Its DR ruined the two-weapon fighting of the barbarian and made the rogue almost incapable of dealing damage except with magical powers. And we had one fewer person to save entombed allies. On paper, this was a great monster, and it was still a fun fight. But it was a bad match with this party and its current capabilities.
Here’s a story I promised to tell. Also in Libris Mortis, there’s a creature called the entropic reaper. It has fast healing, damage reduction beaten only by cold iron and lawful, spell resistance, normal undead resistances, and the ability to deal an average of 100 damage on a critical hit (which my sorcerer took). These alone are pretty scary for a CR 12 creature, but its namesake ability is its entropic blade.
If entropic blade hits you, you make a Fortitude save. If you fail, you’re done in the fight. You are unable to cast spells, take a -4 penalty to attack rolls and a 50% miss chance besides, and have a random chance of targeting allies since you can no longer tell who your enemies are. But each round you’re in this state, you take 1 Wisdom drain, which kills you if your Wisdom reaches 0. The only way to stop this effect is the application of sufficiently powerful healing magic (restoration or heal, which only clerics have at level 12), so if you want to survive, you need a cleric who has one of these spells prepared or an ability to cast one of them spontaneously, such as from a scroll. You can delay the effect, but only for one minute with a DC 21 Charisma check as a standard action, which requires a Charisma of 30 to get to a 50% chance of success. A character with a good Charisma can stave this off almost forever by doing nothing but making this check, but it means they can’t affect the fight in any way.
When we fought this creature, we had a sorcerer, a rogue, a fighter/bard, a druid, and a cohort cleric who used wands. The only chance we had to survive entropic blade was if the cohort entered combat (which he generally hadn’t done yet in the campaign) and cast restoration. But even with a perfect team, the initial Fortitude save is essentially a save-or-die mechanic unless the cleric happened to choose the correct spells at the beginning of the day.
Is the entropic reaper a bad creature? Probably. I don’t think it should have been published, and I can’t imagine why it was included at CR 12. But that doesn’t mean it’s unbeatable. The right party with the right preparation, a sufficiently high-level party that can resist many of its powers, or an appropriate ability to retreat and strategize can all make the fight balanced if not easy. But the lesson here isn’t that some creatures are bad (that’s a different lesson). It’s that CRs aren’t hard and fast representations of the difficulty of fighting a creature. Every monster, challenge, story, and part of a campaign needs to be measured against the capabilities of the characters and the players to challenge them but keep each side of the table from invalidating the other.
This is a lot easier in 4th Edition with its stranglehold on monster attacks, damage, and defenses. But there’s still a lot of room for bad decisions in power design, especially custom power design, and any variance from the expected formula can have hilarious consequences. And any of the non-numeric aspects of a monster can be trouble too; a fire specialist can expect to be sad when a fire elemental comes to a fight, but sending the players on an extended story in the City of Brass is just malicious.