A couple of general notes: In converting creatures, I am generally less worried about an exact conversion and more about being faithful to the concept and getting the CR about right.
Also, vis-a-vis naming conventions, you'll note that I did an "Air Elemental Elder" and not an "Elder Air Elemental". This is organizational- when I alphabetize things, it helps keep like with like.
Obviously, in converting traps and the like, I am taking some liberties. The 3e rules were very different in a lot of ways; the prevalence of ability damage, for instance. That has to be revised to be more in line with 5e rules. Or energy drain- I would replace that with life drain. And so on.
Anyhow, on to last night's work:
5. To comprehend the mad scribbles, a character must make three consecutive DC 25 Int (Arcana or Investigation) checks.
A creature that reads the mad scribbles must make a DC 17 Int save or be confused for 1 minute and take 6d10 psychic damage.
A creture that reads the scribbles gains advantage on dispel and counterspell checks, and other creatures can’t benefit from the Magic Resistance trait against its spells.
6. The Int (Arcana) check DCs are 10 and 20. Determining that the wall is a wall of stone requires a DC 15 Int (Arcana) or DC 20 Int (Investigation) check.
7B. Wis save (DC 16) to resist the room’s effects.
8. See Appendix 1 for the hetfish swarms.
Hetfish Swarm
Large swarm of Tiny monstrosities
Armor Class 18
Hit Points 65 (10d10+10)
Speed Swim 80 ft.
STR 2 (-4), DEX 20 (+5), CON 12 (+1),
INT 2 (-4), WIS 10 (+0), CHA 3 (-4)
Damage Resistances bludgeoning, piercing, and slashing
Damage Immunities fire
Damage Vulnerabilities cold
Condition Immunities charmed, frightened, grappled, paralyzed, petrified, prone, restrained, stunned
Senses darkvision 60 ft., passive Perception 10
Languages -
Challenge 5 (1,800 xp) Prof +3
Swarm. The swarm can occupy another creature’s space and vice versa, and the swarm can move through any opening large enough for a Tiny hetfirsh. The swarm can’t regain hit points or gain temporary hit points.
ACTIONS
Bites. Melee Weapon Attack: +8 to hit, reach 0 ft., each target in the hetfish’s space. Hit: 12 (2d6+5) fire damage, and the target must make a DC 12 Dex save, catching fire on a failure, taking 1d6 fire damage at the start of each of its turns. These flames are supernatural in nature, and can’t be extinguished as long as a creature remains in the hetfish swarm’s space except by magic (including a dispel magic). A creature that ends its turn out of the hetfish swarm’s space can extinguish the flames normally, and if it is underwater, they go out automatically.
10A. A DC 20 Int (Investigation) check allows a pc to discern the frescoes’ faded image.
12. The spears attack at +12 to hit. Each does 1d6 piercing plus 4d10 poison damage. A hit creature must make a DC 17 Con save, taking ongoing 2d10 poison damage (save ends) on a failure.
13. See Appendix 1 for the air elemental elder.
Air Elemental Elder
Huge elemental (Air), usually neutral
Armor Class 16
Hit Points 204 (24d12+48)
Speed 10 ft., fly 90 ft.
STR 22 (+6), DEX 22 (+6), CON 15 (+2),
INT 6 (-2), WIS 10 (+0), CHA 6 (-2)
Damage Resistances lightning; bludgeoning, piercing, and slashing
Damage Immunities poison, thunder
Condition Immunities exhaustion, grappled, paralyzed, petrified, poisoned, prone, restrained, unconscious
Senses darkvision 60 ft., passive Perception 10
Languages Auran
Challenge 14 (11,500 xp)12/13Prof +5
Air Form. The elemental can enter a creature’s space and stop there. It can move through a space as narrow as 1” without expending extra movement to do so.
ACTIONS
Multiattack. The elemental makes two Thunderous Slam attacks.
Thunderous Slam. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 24 (4d8+6) thunder damage.
Whirlwind (recharge 5-6). Str save: DC 19, each Large or smaller creature in the elemental’s space. Failure: 50 (8d10+6) thunder damage, and the elemental pushes the target up to 30’ and knocks it prone. Success: Half damage only.
14. Arly the Weaver uses the following stat block. Remove his possessions but not the trapped Treasure items.
Arly the Weaver
Large fiend, chaotic neutral
Armor Class 17
Hit Points 191 (20d10+80)
Speed 30 ft., fly 30 ft.
STR 18 (+4), DEX 10 (+0), CON 19 (+4),
INT 17 (+3), WIS 18 (+4), CHA 20 (+5)
Saving Throws Str +8, Dex +4, Con +8, Int +7, Wis +8, Cha +9
Skills Arcana +7, Deception +9, History +10, Perception +8
Damage Resistances cold
Senses darkvision 60 ft., passive Perception 18
Languages Common, Abyssal, Giant
Challenge 12 (8,400 xp)Prof +4
Regeneration. The oni regains 10 hit points at the start of its turn if it has at least 1 hit point.
ACTIONS
Multiattack. Arly makes two Claw or Nightmare Ray attacks. He can replace one attack with a use of Spellcasting.
Claw. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 13 (2d8+4) slashing plus 18 (4d8) psychic damage.
Nightmare Ray. Ranged Weapon Attack: +x to hit, range 60’, one target. Hit: 21 (6d6) psychic damage, and the target is frightened until the start of Arly’s next turn.
Shape-Shift. The oni shifts into a Small or Medium Humanoid or a Large Giant, or it returns to its true form. Other than its size, its statistics are the same in each form. Equipment doesn’t transform.
Spellcasting: Arly casts one of the following spells, using Charisma as his spellcasting ability (save DC 17):
At will- darkness, detect magic, invisibility, mage hand, mending, minor illusion.
3/day- comprehend languages, detect thoughts, magic missile, message, unseen servant.
1/day- charm monster, cone of cold, gaseous form, identify (as an action), sleep.
REACTIONS
Shield (3/day). When Arly is hit by an attack or magic missile, he gains a +5 bonus to AC until the end of his next turn and negates the triggering magic missile damage.
