Hello, I want to create a corrosive mutant and I needed help foe the CR of the creature, abilities and balance the creature.
I want to create a character similar to Geiger (comics) or Doctor Phosphorus.
The mutant glows neon green and his skeleton is visible through his skin.
The idea is to obtain similar abilities from the corrosive body spell.
Corrosive body :
You exhale corrosive gas, and acidic secretions coat your skin as you transform into a being of living acid. You gain acid immunity. Any creature that touches you or damages you with an unarmed melee attack or non-reach melee weapon takes 3d6 acid damage, and your unarmed attacks deal 1d4 additional acid damage. The first time each round that you deal acid damage to a creature in this way, you gain 3d6 temporary HP as your body digests the eroded material and transforms it into a protective acid. When the spell ends, any remaining temporary HP expires as well. Your acid spells deal one additional die of acid damage (of the same damage die the spell uses). You can cast acid splash as an innate spell; the splash damage affects all creatures within 15 feet instead of the normal 5 feet.
Withering grasp
Your touch rots organic material and decays objects. Make a spell attack against the target’s AC. Your touch deals 1d12 void damage plus 1d4 persistent void damage. If a creature uses an item to block withering grasp, such as with the Shield Block reaction, the item is automatically affected, but the creature doesn’t take damage (even if there is damage left over after the shield’s Hardness). Unlike normal void damage, the void damage from withering grasp damages objects, constructs, and the like by eroding away their substance.
Critical Success Your touch deals double damage (both initial and persistent). If you target an object, lower its Hardness by 4 for 1 minute.
Success Your touch deals full damage. If you target an object, lower its Hardness by 2 for 1 minute.Heightened (+1) The damage increases by 1d12 and the persistent damage increases by 1. If you target an object, lower its Hardness by an additional 1 point on both a success and a critical success.
Mana Wastes Mutant
All Mana Wastes mutants gain darkvision (replacing low-light vision if the creature had it), and the DC to Recall Knowledge about a mutant increases by 2. The creature gains the mutant trait. They usually have one or two of the following abilities, or potentially three if the mutant is 9th level or higher. If you apply these abilities to an existing creature, either raise the level and adjust the statistics, or remove some of the creature's abilities to make room for the new abilities.
Select two of the following abilities, or potentially three if the Mana Wastes mutant is 9th level or higher.
Caustic Pustules Whenever the mutant takes piercing or slashing damage, creatures adjacent to the mutant take 1d6 acid damage (basic Reflex save). The damage increases to 2d6 if the mutant is at least 8th level and 3d6 if the mutant is at least 15th level.
Chameleon Skin The mutant can change their coloration to match their surroundings. They don't need cover or concealment to attempt to Hide with a Stealth check.
Eldritch Attraction (enchantment, mental) A creature who casts a spell within 30 feet of the mutant must succeed at a Will save or spend its next action to Stride toward the mutant. It can't Delay or take any reactions until it has obeyed the compulsion.
Energy Blast[two-actions] (evocation, primal) Choose an energy type; this ability gains that energy type as a trait. The mutant gains some sort of physical alteration to their shape that suggests how they trigger their energy blast, be it numerous pustules on their body, an extra limb, or an additional opening in their throat to exhale the blast as a breath weapon. Choose whether the mutant creates a 60-foot line of energy or a 30-foot cone of energy when they activate this ability. The energy deals 1d6 damage per level with a basic Reflex save. The mutant can't use Energy Blast again for 1d4 rounds.
Energy Resistance The mutant has strangely colored skin or fur and gains resistance to a single type of energy damage. The amount of energy resistance gained depends on the mutant's level.
Grasping Tentacle The mutant grows a writhing, serpentine appendage, granting them a tentacle Strike (an agile unarmed attack that deals bludgeoning damage), and the Grab ability with that Strike. If they had any agile attacks, the damage dealt by their tentacle should be roughly the same as the damage dealt by those attacks. If they had only non-agile attacks, their tentacle should deal three-quarters that damage.
Hulking Form Increase the mutant's size by one category. Decrease their AC by 1, increase their HP by 1/10 their normal maximum HP, and increase the reach of all their melee Strikes by 5 feet.
Hungry Maw A mouth full of razor-sharp teeth opens somewhere on the mutant's body. They gain a maw Strike (an unarmed attack that does piercing damage roughly the same as the damage dealt by their most damaging melee attack), and they gain the Vengeful Bite reaction.
Vengeful Bite [reaction] Trigger A creature critically hits the mutant with a melee unarmed Strike or a non-reach melee Strike; Effect The mutant makes a maw Strike against the triggering creature.Increased Speed The mutant's limbs change, or they grow new limbs to accommodate their mutation. Either one of their existing Speeds increases by 20 feet or they gain a new Speed (such as fly or swim) of 15 feet.
Magic Hunger The mutant gains the ability to detect the source of any magic within 30 feet as an imprecise sense.
Mirror Thing [one-action] (arcane, concentrate, polymorph, transmutation) The mutant takes on the specific appearance of any Small or Medium humanoid they can currently see. This doesn't alter their statistics, but it might change the damage type of their Strikes.
Revolting Appearance (arcane, aura, visual) Details of the mutant's appearance are disturbing and unnatural in the extreme. A non-mutant living creature who begins their turn within 60 feet of the mutant must succeed at a Fortitude save or become sickened 2. A creature who succeeds at its save is then temporarily immune to the aura for 24 hours.
Sprouted Limb The mutant grows an extra leg, increasing their Speed by 10 feet.
Too Many Eyes Countless eyes sprout on the mutant's skin. They gain all-around vision.
Afflicting Strike The mutant must have a jaws, claw, sting, or other melee Strike to gain this ability. That Strike now inflicts a disease, poison, or similar affliction on a hit, with a Fortitude save to resist. You can use the rules for existing monsters of the mutant's level for diseases, poisons, and similar afflictions to give to your mutant, or you can simply say the strike inflicts an additional 1d6 poison damage + an additional 1d6 points per 4 mutant levels on a hit, with a basic Fortitude save. This makes the Strike significantly more deadly, so reduce the damage in other ways to account for this increase.
Energy Resistance The mutant has strangely colored skin or fur and gains resistance to a single type of energy damage. The amount of energy resistance gained depends on the mutant's level:
Energy Blast [two-actions] (evocation, primal) Choose an energy type; this ability gains that energy type as a trait. The mutant gains some sort of physical alteration to its shape that suggests how it triggers its energy blast, be it numerous pustules on its body, an additional limb, or additional opening in its throat to exhale the blast as a breath weapon. Choose whether the mutant creates a 60- foot line of energy or a 30-foot cone of energy when it activates this ability. The energy deals 1d6 damage per level with a basic Reflex save. The mutant can't use Energy Blast again for 1d4 rounds.
Inoculation The mutant's flesh is distorted with the addition of unexpected fur, scales, warts, or other growths. The creature gains a +2 status bonus to saves against disease and poison. In rare cases, you might choose to grant immunity instead.
Increased Speed The mutant's limbs change, or it grows new limbs to accommodate its mutation. Either one of its existing Speeds increases by 20 feet, or it gains a new Speed (such as fly or swim) of 15 feet.
Creating Mutants
In most cases, it's better to build a new Mana Wastes mutant from the ground up, using the standard rules for monster creation (this is how the mutants presented here were built). An existing creature can be turned into a Mana Wastes mutant by increasing its level (generally by 1 or 2), increasing its statistics accordingly, and giving it one or more of the Mana Wastes mutant abilities on this page.
quirk (deviant abilities)
strange biology :
Glows neon green and his skeleton is visible through his skin.
0/ The creature can't hide or sneak, a reduction for the CR ? An existant creature with a similar abillity ?irradiate (2 actions)
You exude an aura of radiation, making everyone around you ill. All creatures in a 15-foot emanation must succeed at a Fortitude save or become sickened 1. On a critical failure, they’re also fatigued for 1 minute. You’re immune to your own radiation. The value of this sickened condition increases by 1 for every 5 levels you have beyond 6th.
Awakening You become more efficient at exuding this aura of radiation. The radius of the emanation becomes 60 feet.
Awakening Your radiation becomes more potent. Creatures in the area take an additional 1d4 poison damage for every 2 levels you have, with a basic Fortitude save (roll this Fortitude save once and apply it to both the damage and the sickened condition).
Unleash the blight (2 actions)
You unleash the rot infecting you, spreading the blight to those nearby. All living creatures in a 60-foot emanation take 1d6 void damage for every 2 levels you have, with a basic Fortitude save.
Awakening The blight is infectious. Creatures damaged by your blight take an additional 1d6 persistent void damage, as they begin to decompose.
Awakening The blight is debilitating. Creatures damaged become drained 1 for one round on a success, drained 1 for 1 minute on a failure, and drained 2 for 1 minute on a critical failure.
Fen pudding CR 12
Corrosive Mass A creature that hits the pudding with a metal or wooden weapon must succeed at a DC 22 Reflex save or the weapon takes 2d6 acid damage (after dealing damage to the black pudding as normal). Thrown weapons and ammunition take this damage automatically with no save.
Verdurous Ooze CR 7
Corrosive Surface A creature that hits a verdurous ooze with a metal weapon or unarmed attack must attempt a DC 21 Reflex save. On a failure, the weapon or creature takes 2d4 acid damage (after dealing damage to the ooze as normal). Thrown weapons take this damage automatically with no save.
Corrosive Touch When the pudding hits a creature with its pseudopod, any acid damage is dealt to the creature's armor or clothing as well as the creature.
Empress Bore worm CR 7
Painful Bite The bite of an empress bore worm causes excruciating pain. The target must succeed at a DC 25 Fortitude save or become sickened 1 from the pain (sickened 2 on a critical failure).
Ba'aupa Mdoudu CR 13
Slime Squirt [reaction] Trigger A creature infiicts slashing or piercing damage on the ba'aupa mdoudu with a melee Strike; Efiect The attacking creature takes 2d10 acid damage.
Caustic Acid Whenever a creature takes acid damage from any of a ba'aupa mdoudu's abilities or Strikes, it must succeed at a DC 30 Fortitude save or become drained 1 (drained 2 on a critical failure). On each subsequent failure, the drained condition value increases by 1 (or by 2 on a critical failure), to a maximum of drained 4.
Fleshforged Dreadnought CR 18
Caustic Leak [free-action] (acid, arcane, evocation) Frequency once per turn; Trigger The fleshforged dreadnought is damaged by a non-reach piercing or slashing melee Strike; Effect Corrosive fluid sprays the attacker, dealing 8d6 acid damage (DC 40 basic Reflex save).
Somnalu CR 15
Corrosive Wounds The acid that coats the skin of Somnalu eats away at a weapon that Strikes it. As long as Digestive Secretions is active on Somnalu, whenever a creature hits Somnalu with a metal or wooden weapon, the creature must succeed at a DC 33 Reflex save or else the weapon takes 2d8 acid damage (after dealing damage to Somnalu as normal). Thrown weapons and ammunition take this damage automatically with no saving throw.
Baleful Gaze [reaction] Trigger A creature within 100 feet makes a ranged attack or uses an action that has the concentrate trait; Effect Somnalu fixes its central eye upon the triggering creature in an attempt to frighten it, jolting it and disrupting its action. The creature must attempt a DC 36 Will save; on a failure, Somnalu disrupts the triggering action and the creature is frightened 1.
Drain Nutrients [two-actions] Somnalu targets a creature with a claw Strike. On a success, Somnalu drains the creature's life through its skin. The creature must succeed at a DC 36 Fortitude save or become drained 2 (drained 1 on a success, no effect on a critical success). Somnalu gains temporary Hit Points equal to the amount of Hit Points the creature loses from this Strike.
1/ How do calculate DC for the saves, damage with the level of the creature ?
2/ Help for abilities and the choices.
Caustic pustules similar to corrosive mass/surface/touch/wounds ? Caustic leak ?
Optional : sickened, drained condtions from painful bite/caustic acid.
hungry maw similar to slime squirt ?
Afflicting strike similar to drain nutrients ?
Energy blast similar to irradiate, unleash the blight ?
Hungry maw or eldritch attraction similar to baleful gaze ?
Thanks for your future help.
Most of this is going to have guidelines in the Building Creatures rules. Especially the section on Design Abilities.
1/ How do calculate DC for the saves, damage with the level of the creature ?
Use the tables 'Area Damage' and 'Spell DC and Spell Attack Bonus'.
2/ Help for abilities and the choices.
First, don't put a lot of abilities onto one creature. They won't have the time to use all of them in a fight. You could either create a family of these creatures where different ones have their own subset of abilities, or have a collection of abilities that each instance of creature on the map gets to choose one or two of the abilities.
Caustic Pustules: I would use the examples of Slime Squirt or Caustic Leak. The permanent action-free retaliation examples from oozes are not balanced correctly for non-ooze creatures. So make the ability cost a reaction or a free action with a frequency limit.
For other effects on a strike: Use the examples of Grab and Shove, and Greater Constrict and Rend creature abilities. For debuff abilities, it costs an additional action after a successful Strike and then applies the debuff. For damage dealing abilities it should cost two actions, or one action after two successful Strikes to apply additional damage.
For effects not tied to a Strike: Generally those are going to cost two actions. Like spells.