Beithir
Beithir — The Encounter
The Lightning-Serpent of the Highland Glens, Greatest of Venomous Worms · a boss your table can run with almost no prep. SRD 5.1-compatible.
Who Is Beithir
In the old tongue of the Highland glens the beithir is named greatest of the serpent-kind, a venomous worm grown vast in lochs and corrie-hollows where no shepherd dares graze. The grimoires hold that it is born of a common adder that survives a hundred years and a hundred storms, each thunderclap swelling it until horns sprout and its hide bristles with spines. It carries the lightning that made it, and a strike of the sky's fire only feeds the thing. The old wisdom warns the traveller never to flee in a straight line, for the beithir runs swift over heather and stone, but to cross running water, which it will not pass without coiling thrice and screaming. A severed beithir must be burned at once, head laid far from tail, lest the two creep together in the dark and the worm rise whole again before morning bell.
Run Beithir in Sixty Seconds
- Lock first. Constrict before you Bite — a grappled, restrained PC can't easily run, and that's where all the worm's pressure comes from.
- Kill-priority = the grappled target. Venom-Slick Coils stops them regaining hit points, so focus Bites on whoever is pinned and let the party scramble to free them.
- Feed it lightning at your peril. Storm-Wakened turns any lightning damage into temp HP plus a bonus-damage bite — warn lightning-casters, and if YOU run a storm boss alongside, zap the worm on purpose.
- When bloodied, have it dive to swim/climb out of melee, drag its grappled victim along, and use Tail Lash on the chaser — make the party split to rescue.
- Three ways to run it: glen ambush from a loch (blindsight in murk), coiled lair-guardian walling off a vault, or storm-fed war-beast beside a cult boss who keeps it charged.
Beithir
Huge monstrosity, neutral evil
CR 6 · 2,300 XP · GUIDELINEVenom-Slick Coils. When the beithir hits a creature it has grappled with a bite or constrict, that creature must succeed on a DC 15 Constitution saving throw or be poisoned until the end of its next turn. While poisoned this way, it can't regain hit points.
Storm-Wakened. Signature. The beithir was hatched in a thunderstorm and carries its charge. The first time each turn it takes lightning damage, it instead gains 10 temporary hit points, and its next bite this turn deals an extra 7 (2d6) lightning damage.
Actions
Multiattack. The beithir makes one Bite attack and one Constrict attack.
Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 15 (2d10+5) piercing damage plus 7 (2d6) poison damage.
Constrict. Melee Weapon Attack: +8 to hit, reach 10 ft., one Large or smaller creature. Hit: 14 (2d8+5) bludgeoning damage, and the target is grappled (escape DC 15) and restrained until the grapple ends. The beithir can constrict only one creature at a time.
Bonus Actions & Reactions
Tail Lash. Reaction (when a creature the beithir can see ends its turn within 10 ft. of it): The beithir whips its spined tail at that creature. The target must succeed on a DC 15 Dexterity saving throw or take 9 (2d8) slashing damage and be knocked prone.
A great fanged worm rears from its own coil, spined head crowned with horns, foreclaws raised and a barbed tail sweeping the wet stone of a Highland glen.
Beithir, Bound
Companion · acts on your initiative
Bound · GUIDELINECoil and Constrict (signature). Beithir can grapple a Large or smaller creature with its body (escape DC 13). While it grapples a creature, its bite deals an extra 4 (1d8) poison damage.
Actions
Bite. Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: 11 (2d6+4) piercing damage plus 4 (1d8) poison damage.
Reactions
Slip the Strike. When a creature within 10 ft. is hit by an attack, Beithir lunges between, taking the damage in the ally's stead and then making a Bite attack against the attacker if within reach.
A lesser worm of the same brood, scarred and storm-marked, that has chosen a master and coils ever at their heel.
Guideline numbers — formula-tuned and read from the miniature's design, not yet table-proven. Tell us how it ran in the comments.
How It Fights — by Tier
Same creature, scaled threat. The traits never change; only the numbers move.
Open with Constrict on the front-line fighter to lock them in place, then Bite the grappled target every turn so the poison keeps healing off. Save Tail Lash for whoever steps adjacent. At this scale it's a true setpiece worm — give it a coiled-around-terrain pit so melee must choose between freeing the grappled ally and reaching the head.
Open by Constricting the squishiest reachable PC and dragging the fight to the worm; Venom-Slick Coils shuts off their healing while restrained, so that's your kill-priority lock. Tail Lash punishes anyone who closes in. Don't sit still — slither (climb/swim) to break line of sight from casters. Pairs well with a venom-priest or storm-caller boss who feeds it lightning to trigger Storm-Wakened.
Run it as the rampart of a lair or as a guardian-beast paired with a bigger threat, not a solo boss — a full party out-acts a single worm. Lead with Constrict to pin the healer, Bite to suppress their healing, and use the coil to wall off the room. Have lesser adders or cultists feed it lightning so Storm-Wakened keeps the temp HP and bonus damage flowing.
Put Beithir on the table
Download the presupported STL and run the fight tonight — the model, and the encounter that comes with it.
Get the STL on MyMiniFactory →Clay Cyanide Companion Rules v1
How a bound creature acts
- A companion acts on its owner's initiative, immediately after the owner.
- It uses the Companion block above — roughly ×0.6 HP and damage, no recharge abilities, and at most one reaction trait.
- It moves freely; it takes its Action only while its owner is conscious.
- At 0 HP a companion is broken, not dead — restored on a long rest unless your table rules otherwise.
Original Clay Cyanide material © Clay Cyanide LLC — explicitly not the non-SRD official sidekick rules.
Questions, Asked at the Table
What CR is Beithir?
Natively CR 6 (Tier 2, ~2,300 XP) — a Huge venomous worm built as a front-line grapple-bruiser. The Tier 1 and Tier 3 columns rescale the same traits to CR ~2 and ~11 so you can drop it into any party level without rewriting its kit.
Can Beithir be a companion or ally?
The adversary is too large and savage to befriend, but the companion block gives you Sgian, the Bound Adder — a lesser worm of the same brood (AC 14, 82 HP) that grapples, bites with poison, and can throw itself in front of an ally's hit. Use it as a druid/ranger beast-bond, a beast-master's pet, or a cultist's guard-serpent.
Why isn't the beithir a solo boss?
It's a powerhouse, but its kit is a single grapple-and-bite loop with no legendary actions or initiative-stealing reactions, so a four-PC party will out-act it across a round. Run it as a centerpiece add-on — a lair guardian or storm-fed war-beast beside a true boss who can feed it lightning.
Tell Us How It Ran
This work includes material taken from the System Reference Document 5.1 ("SRD 5.1") by Wizards of the Coast LLC and available at dnd.wizards.com/resources/systems-reference-document. The SRD 5.1 is licensed under the Creative Commons Attribution 4.0 International License available at creativecommons.org/licenses/by/4.0/legalcode.
Clay Cyanide Miniatures is not affiliated with, endorsed, sponsored, or specifically approved by Wizards of the Coast LLC. This content is compatible with the fifth edition of the world's most popular fantasy roleplaying game. Companion rules, lore, names, and original mechanics © Clay Cyanide LLC.
Technical Specifications
32mm, 75mm
STL, Pre-supported STL
Standard or ABS-like resin, 0.05mm layer height
Yes — ready to print, optimized supports included
Played Beithir at your table? Log in with your Obsidian Tavern account to leave a balance note, a war story, or a file issue — the sharpest reports get the block revised.
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