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