Converting Oonga from "Isle of the Ape" to 5e

Question

My party of five level 15 PCs is adventuring in my homebrewed conversion of the 1e module Isle of the Ape. I would like the Boss Monster, Oonga, to be a Deadly encounter for the party, so I am aiming at a CR of 23 or 24 while keeping him faithful to the source material.

I have seldom played and never designed an NPC for this level of play and would appreciate feedback on the appropriateness of how I have statted him out. A helpful answer would both 'check my math' and provide some insight into whether this monster would 'play out at its CR' or have reasons for not doing so.

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Oonga

Gargantuan Beast, unaligned
AC 19 (natural armor, thick skin)¹
HP 559 (26d20 + 260)²
Speed 60 (Climb 60)

Str 28 (+9) Dex 14 (+2) Con 30 (+10) Int 8 (-2) Wis 14 (+2) Cha 10 (0)

Saving Throws: Int+5, Wis+9, Cha+7
Skills: Athletics +16, Perception +9

Damage Resistances: Oonga reduces all damage taken from fire, cold, and lightning by -1 per die of damage due to his bulk and toughness.

Damage Immunities: None

Condition Immunities: Charmed, Frightened, Paralyzed, Poisoned. Oonga is also immune to the effects of the Slow spell.

Senses: Passive Perception 19

Languages: -

Proficiency Bonus +7 ¹

Senses: Passive Perception 19

Legendary Resistance (3/Day). If Oonga fails a saving throw, he can choose to succeed instead.

Siege Monster. Oonga deals double damage to objects and structures if large trees are available for him to rip up and use as a club.

Crushing Blow. Any hit from Oonga's melee attacks with the exception of his bite reduce the AC of non-magical armors and shields by 1. Shields are affected first, and are destroyed when their protective value is nil. Armor is destroyed after it has been reduced by a number of hits equal to its (base AC-10)/2.

Regeneration. At the start of his turn if Oonga has at least 420 hp, he regenerates 5hp. If he has at least 280hp, he regenerates 1hp every 10 minutes.

Actions

Multiattack. Oonga can make two fist and one bite attack.

Fist. Melee Weapon Attack: +12 to hit,³ reach 15ft., one target. Hit: 31 (4d10 + 9) bludgeoning damage. If a S or M target is hit with both fists on the same turn, and has appendages that may be grabbed and pulled, Oonga does an additional 11 (2d10) bludgeoning damage.

Bite. Melee Weapon Attack: +12 to hit, reach 10ft., one target. Hit: 10 (4d4) piercing damage.

Rock. Ranged Weapon Attack: +12 to hit, Range 60/360. All targets in a 10' x 10x area (roll damage separately for each hit). Hit: 31 (4d10+9) bludgeoning damage.

Legendary Actions

Oonga can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Oonga regains spent legendary actions at the start of his turn.

Charge. Oonga moves up to 60 feet toward an opponent. If he moves at least 30 feet, he makes an Athletics check contested by the opponent's Athletics or Acrobatics. If he wins, the opponent takes 26 (4d12) bludgeoning damage, is knocked prone, and Oonga may then make an additional bite attack.

Stomp. Oonga makes a stomping attack. Melee Weapon Attack: +12 to hit, reach 5ft., one or two different targets of size M or smaller. Hit: 13 (2d12) bludgeoning damage, rolled separately for each target hit.

Grasp and Hurl. Oonga attempts to grapple a single opponent of size Small or Medium. He makes an Athletics check contested by the opponent's Athletics or Acrobatics. If successful, he first crushes the target for 20 (2d10+9) bludgeoning damage and then lifts them up and hurls them to the ground for 44 (10d6+9) bludgeoning damage and they land prone.

Calculating Challenge Rating

Defensive:
¹AC for CR24 is suggested to be 19
²Hp. for CR24 are suggested 536-580
Defensive CR24

Offensive
The suggested DPR for CR24 is 195-212. I am calculating Oonga's maximum average DPR as 209.
I understand that DPR for CR assumes most effective attacks and that they always hit. But in this particular case I wonder about all the contingencies implied. 31+31+11+10+26+10+13+13+20+44 = 209 assumes that Oonga will hit the same target with both fists and thus be able to rend, will bite, and over the course of three rounds will be able to use each of his legendary actions each round, including charging one opponent, stomping on two different opponents, and grasping and hurling one opponent. I suppose since the grasp and hurl attack is the most damaging, I might be meant to calculate the DPR as being from three grasp and hurls per round for three rounds? (This would add 83 to his DPR to make the total 292, which pushes the CR for DPR all the way up to 29). Or, I could just add the language that some legendary creatures have that they cannot use the same legendary action twice in a row (or in his case more than once a round?).

³After DPR is proficiency bonus, which is +7 as suggested for CR24. Note that I am keeping hit attack bonus as +12, also suggested by CR24, rather than the +16 it would be for his Str mod plus proficiency bonus.
Offensive CR24

Average CR24.

Answer 1

Looks like a careful conversion, likely should be CR 27

5e monster building CR

Defensive CR:

HP look OK. "Regeneration" advises you to increase the monster's effective hp by 3x the number it regenerates each round, which would be 15, and by itself would not change the CR bracket. Legendary Resistance adds 30 hp per use, so 90 overall, pushing it to 674 hp, or CR 27.

AC is straightforward and still 19 at CR 27, so no change.

Offensive CR: For the damage output, you would calculate it per round. The bite lacks the Strength damage bonus. Typically it is added to bite attacks. The same for Stomping attacks, they also normally include the strength bonus.

He can do a multiattack for 31 + 31 + 11 (rend) and 19 (with Str) bite for 90 points.

He has 3 legendary actions per round and is not forced to cycle through them the way you wrote it. Assuming he always picks the one that does most damage, this would always be Grasp and Hurl, even if the stomp deals 2d12+9 (22) damage and hits two targets. As you use the raw damage without adjusting for to hit chance, you ignore the Atheletics check for this to connect. With Athletics +16, this attack by the way would be more likely to hit than a normal attack against most PCs: even a Str 20 proficient PC would have only +10 on Athletics, and with an average roll by the PC of 11, the number to reach for Oonga would be 21, at +16 probably easier than reaching the PC's AC with +12.

So the 3 legendary actions give you 3 x (20 + 44) = 192 damage.

Total damage per round: 282 points, or CR 28.

This would imply an attack bonus of +13, the +12 you have does not alter this as it is not two steps off. This would make his offensive CR 28.

Total average CR: (27 + 28) / 2 = 27

5e design sensibilities

My experience converting 1e monsters into 5e, is from doing so for all the ones in GDQ1-7 Queen of Spiders that did not exist in the original 5e Monster Manual. That does not mean I did a good job of it, but some were later published in other compendia, and it was quite close.

I think that you are hewing to close to the mechanisms of 1e in your attempt to do the original justice. 5e is expressing many mechanics differently, and you can keep the spirit of the original while sticking to the design sensibilities of 5e (I list the original Oonga below, for comparison). Observations:

• In 5e, damage resistance is a standard mechanic, use it instead of the special -1 adjustment. He has no resistance to bludgeoning, slashing or piercing, and over CR 17, other damage resistances come free when it comes to CR.

• Ranged Attack: If you want him to be a challenge for a high level party, you have to increase the range of the Rock attack to beyond longbow range of 600 feet. Otherwise, a flying Sharpshooter will just pick him apart with his longbow at no risk. 5 points of regeneration cannot keep up with the expected damage output. (At +5 proficiency, +2 archery fighting style, +5 dex = +12 to hit vs AC 19 or 70% hit chance at d8+5 damage = 6.875 damage per round, even without any magical buffs or weapon.)

• The original Magic Resistance was a lot stronger than the new one. At 100%, Oonga was pretty much immune to spells. This is hard to deal with - 5e normally does not do this. Iron Golems and even the mighty Tarrasque do not have this kind of immunity to magic. You probably should give him the normal 5e magic resistance. It would increase effective AC by 2. you might also want to give him Immutable Form like the Iron Golem, to avoid him being polymorphed, which will not affect CR.

• Siege Monster. The typical way to do this is just to say "[Monster] deals double damage to objects and structures."

• Slow immunity. I think it is fine to keep this. The One D&D playtest is adding Slowed as a condition.

• Regeneration you already upped it to better match the larger hit point pool and damage output of 5e, which I think is good. I would drop the complications with the faster and slower regeneration at different hp levels (granted, it is like that in the original, but 5e tries to simplify mechanics).

• Crushing Blow I assume this aims to reflect the language that Whenever a blow, foot stamp, or grapple takes place, all possessions, except weapons, must be saved for vs. crushing blow. It seems similar to Rust Monster's Rust Metal and Antennae, and a fair way to convert this. The original is obviously a lot more brutal.

• Ability scores As mentioned above, Strength normally would also apply to the bite. Instead of cutting it out selectively there, if your goal is to keep things around CR24 (as mentioned in a comment), then you could peg it lower, which will bring the damage from all the attacks down. Likewise effective hp are above CR24, and one way to address this is to lower Constitution somewhat.

Deadly Encounter

If you go by the book, for 5 x 15th level the "Deadly" encounter budget (p. 82 DMG) would be 32,000 XP, which would only be a CR 21 monster (33,000 XP). However, a "Deadly" encounter is not really deadly. It's defined as

A deadly encounter could be lethal for one or more player characters.

so it is not a "Flee, you fools!" experience for the party, and not even a 50/50 toss up. Someone could die. Our play experience at similar levels has shown that the XP budgets for deadly encounters are way below what a party can handle. So while the DMG says "When putting together an encounter or adventure, especially at lower levels, exercise caution when using monsters whose challenge rating is higher than the party's average level." (p. 82), that is not true at higher levels, and you seem to have come to similar conclusions by putting the CR at 24.

To live up to the spirit of the original Oonga, it should probably be at a difficulty level that approaches TPK. Here is a quote from Gary himself:

after finally roughly handling the native population (...) the crew go into the island's interior. One PC, and I won't say which, met Oonga, got grabbed, wrenched, bittem [sic], thrown down, and then drop-kicked off the ledge of the cave. A wish was expended to save him. That ended all interest in further adventuring there :(

So if anything, don't be afraid to go overboard, CR 27 should be just fine.

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Oonga (1st Edition)

FREQUENCY Unique
NO. APPEARING: 1
ARMOR CLASS: 2 \ MOVE: 18” + 6“ charge
HIT DICE: 24 + 4
HIT POINTS: 288 hp
IN LAIR: 100% unless encountered randomly
TREASURE TYPE: Special
NO. OF ATTACKS: 3 + 2
DAMAGEIATTACK: 1d12 + 12 (x2), 4d4, 5d4 (x2)
SPECIAL ATTACKS: See below
SPECIAL DEFENSES: See below
MAGIC RESISTANCE: 100%
INTELLIGENCE: Low to average Low to average
ALIGNMENT Neutral
SIZE: L, 30 ft. tall
PSIONIC ABILITY Nil
LEVEL/XP VALUE: 31,880

Oonga travels rapidly, upright and then on all four, according to his desire. When charging, he can leap 60 feet upward (total upward reach) or across.
Attack is by a pair of clubbing fist blows and a bite. Oonga’s great strength gives a bonus of + 12 points of damage to the former sort of attack mode, and if both fists strike the same opponent, then this indicates the mighty simian has grasped the creature and pulled a wrenched it. Such rending inflicts an additional 1-20 points of damage to the victim.
If the monster charges and strikes, he has grappled with the opponent, and there is a 50% chance it has been overborne. Such overbearing inflicts 2-20 points of damage and enables the ape to automatically inflict biting damage as well. Of course, Oonga will grapple with opponents which are near his size, or larger. Against small (man-sized or shorter-8 feet or less) foes, the gargantuan ape will also use a stamping attack, each huge foot inflicting 5-20 points of crushing damage against separate opponents. Oonga is also able to effectively uproot trees of 90 foot height or less and 9 feet or less diameter. Against structures such as gates and walls, Oonga’s clubbing attacks and subsequent pushing act as a battering ram (qv., DMG).
Oonga attacks as a 25+ hit die monster (roll to hit scores on the Supplemental Attack Matrix for Monsters on page 47). Because of his size, heavy coat, thick skin, and exceptional constitution, Oonga regenerates 1 hit point per round, until serious damage (more than 72 points) has been accumulated and unregenerated. Such damage is only restored at the rate of 1 point per turn. If more than 144 points of damage is suffered, then Oonga will require 1 day per point of damage above 144 points sustained.
Similarly, successful magical attacks such as fireballs, lightning bolts, and cold- based ones are at -1 per die of damage due to Oonga’s bulk and toughness. His brain and nervous system are such that he can’t be magically charmed, held, or even slowed. Grasping is an attack form which can be applied to any opponent of 8 feet or less height and weighing 500 pounds or less. The mighty ape simply wraps the creature within its grasp and squeezes. This inflicts 24 points of damage on the round grasping and squeezing occurs. The ape will then squeeze and hurl the opponent down to the ground and stamp on the victim, all during the second round after initially grasping. Hurling from a 40-foot height inflicts 10d6 +12 (strength) of damage.
Oonga is also able to hurl boulders or similar missiles 1 to 36" distance. These missiles will hit a target area of approximately 10 feet x 10 feet, and all creatures within the target area will receive damage. Damage is 5d4 + 12 (strength) and found separately for each creature struck.
Whenever a blow, foot stamp, or grapple takes place, all possessions, except weapons, must be saved for vs. crushing blow.