Okay - the Argorian Wormkin appeared in "Desert Light" - a desert-themed collection of monsters as a mini-module idea in White Dwarf 29. I liked them as a hydra-replicating idea and small treasure-horde guarding critter...all comments welcome as happy to tweak with input
For clarity, I'll put my comments in
blue and use
red text for suggested modifications I’m less committed to.
Shouldn't it have the creature's name at the start in a larger bold font rather than "Description"?
Said description could do with a little polish, plus it wouldn't hurt to add a Flavour Paragraph or two.
It doesn't really need the "which they attack with" in the description, since that's implicit in them having a bite attack. Also, I believe the 5E D&D convention is to use US measurements, so it'd be "foot long" rather than "30 cm long".
The "cannot be killed" and/or "splits into two halves" traits can be mentioned in a Flavour Paragraph abilities but really need to be Special Abilities in the creature's statblock. I'll discuss those below.
I'd suggest something like the following for the Description:
Argorian Wormkin
These foot-long creature where first produced by an evil cleric-assassin from Argor. They are worm-like creatures resembling a reddish pink lamprey with a sucker-like rasping mouth.
Treasure Guardians. These worm-constructs were originally created to protect their master's treasure and are instinctively attracted to shiny valuables, which they rest beside while waiting for intruders to attack.
Immortal Worms. Argorian wormkin are not truly alive and do not need to eat, drink, or drink. They can fully recover from any damage that doesn't kill them and can even survive being torn apart, which usually only separates them into two fragments each capable of attacking.
Stat Block
The Armor Class should have "(natural armor)" rather than "Natural" but the number's fine.
I'd consider possible tweaking the Con up to a +1 bonus since the original had 1d8+1 Hit Dice. Maybe reduce the number of HD to 3d4 to compensate? That'd improve the Poison DC too.
The original monster has Speed 12" which is human average, so why does the conversion move at 40 ft. rather than the 5E average of 30 ft. like a Poisonous Snake? The original monster didn't have a climb speed, but I have no particular objection to it. Personally I'd reduce it to 20 ft. and add a swim speed, but that's mainly because I'm used to 3E snakes and their "Speed: 20 ft., climb 20 ft., swim 20 ft."!
The bite has 5 listed as the average damage but that doesn't match the 2 average of the listed 1d4 damage. Should this be Hit: 5 (1d4 + 3) or Hit: 2 (1d4)?
I'd be inclined to go inbetween and make it Hit: 4 (1d4 + 2) like a Giant Centipede or possible Hit: 3 (1d4 + 1). Also if the mouth is lamprey-like it's not stabbing like a giant spider's fangs but is rasping/slicing at the flesh, so wouldn't it do slashing damage rather than piercing?
Furthermore, while the poison causing them to be unable to do anything except crawl or dodge for 1d4 rounds does match the original monster it (a) seems too nasty for the 5E design philosophy and (b) why not use an existing 5E Condition like incapacitated (for example, Boomslang Venom). I would think the poisoned condition (disadvantage on attack rolls and ability checks) should be enough, but there's nothing wrong with adding the retching/slowness to it. Maybe have multiple doses cause the incapacitating nausea?
As for the Special Abilities, their "Split" ability needs a bit of expansion and clarification. At the moment it appears to say that if you hit one for 10+ damage it's killed, but 1-9 damage just causes it to split into two wormkin each with full hit points (i.e. 10 hp apiece). So, in theory, their creator could have ordered them to continuously attack each other and exponential increase their numbers until there's millions of the blighters. The wormkin only do 1d4 piercing damage so they can't kill each other with their bite.
That sounds a bit unbalanced, so I'd prefer the split to be temporary and have the wormkin still suffer damage from the blow that divided them. I'd suggest modifying the Split ability of an ooze, i.e.:
5E SRD said:
Split. When a jelly that is Medium or larger is subjected to lightning or slashing damage, it splits into two new jellies if it has at least 10 hit points. Each new jelly has hit points equal to half the original jelly’s, rounded down. New jellies are one size smaller than the original jelly.
So something like:
ARGORIAN WORMKIN
Armor Class 14 (natural armor)
Hit Points 10 (3d4 + 3)
Speed 30 ft., climb 20 ft., swim 30 ft.
CON 12 (+1)
Cannot Be Killed. If an Argorian wormkin is damaged but not destroyed, it will fully recover from its injuries in an hour, regardless of the form of damage it took.
Actions
Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 4 (1d4 + 2) slashing damage plus poison, the target must make a DC 11 Con save or be poisoned for 1d8 rounds, during which time violent retching reduces their speed to half (round up to the nearest 10 ft., minimum speed 5 ft.). If a target is poisoned multiple times by wormkin venom, they become incapacitated with nausea while within the duration of two or more doses of poison..
Reactions
Division. When a wormkin is subjected to hit points of damage, it splits into two wormkin if it has at least 2 hit points. Each wormkin has hit points equal to half the original wormkin's, rounded down. The wormkins rejoin into a single wormkin with full hit points an hour later (see
Cannot Be Killed).
It's hard to tell, but I'd eyeball that as maybe Challenge 1/4. It's hard to tell since its Division ability makes calculation difficult but it looks less formidable that the CR 1/2 Gray Ooze but is certainly nastier than the CR 1/8 Giant Rat or Giant Weasel since you may have to kill two or three divided wormkin before it dies.
