DRIDER: Monster Conversion - AD&D 1e to Call of Cthulhu 5e.

Closer to what I'm thinking of than a search for Drider brings up.
Thanks to Arachne, Dante, Dore and Wikicommons for this.

Unless you've already ported Lolth into your Call of Cthulhu campaign (in which case I'm interested in what it looks like now), Driders are 'just' spidertaurs - and the spider part isn't even a spider, really.

Driders aren't a species to themselves, and will be known by chosen/given names or possibly by a local/legendary name for spidertaurs (eg. Jorogumo).

In this conception, Driders have nothing to do with Lolth or the Drow; are no more common in Japan than anywhere else, and have no association with Atlach Nacha or Leng unless they themselves seek it - they can expect no particular favour for similarity of form.

No silk (this is 1st edition AD&D), no Drow spells. Unless you want to.

Drider,  Venom Bloated Composite Multipede.

You become a Drider because you find a way to bind yourself to a giant spider-like body in order to extend your lifespan to study the foul & forbidden, because the Mi-Go do elective/involuntary surgery on you, or because you are struck by a curse. 

You will live well beyond a human lifespan without loss of vigour, as long as you do not succumb to violence or the venom your monstrous body produces. This is ideal for continuing your awful studies, to carry out the will of the Mi-Go, or to prolong your suffering.

Blood is now the perfect food for you, although you can subsist on anything high in protein and/or iron, as long as it’s at least semi-liquid. If you like, you can continue to eat human food for appearance and pleasure, but it is not compatible with your radically altered internal organs and body chemistry - it won't sustain you long term, but it won't do you any harm.

You have fangs, retractable or otherwise, in your human mouth to deliver venom and drain blood. It seems reasonable that you may be able to open your jaw much wider than you used to.

While other configurations are possible, the Drider you are expecting is a spidertaur. The spider section only superficially resembles an arachnid, and could easily have four or a dozen legs as eight. They can be hairy, scaly, chitinous, slimy - anything you like.

The human portion looks as it did in its previous life, although swollen, puffy, discoloured with venom, eventually leaving a permanent mark despite frequent purges (see below). Venom accumulates in the humanoid part at 2 POT/ day, and is uncomfortable to excruciating, with accompanying emotional and mental distress. 

At 12 POT + the Drider will be violently irritable, even destroying allies and important research. 

At 16 POT + it takes a poison attack per day for d6 temporary STR, CON, INT & DEX damage. Once venom is below 16, characteristics recover 1 point per day.

At any time it can purge by biting, ‘milking’ its fangs, or directly tapping its sacs with a blade or syringe - some will have spigots permanently inserted to make this easier. There is bound to be a nefarious text somewhere claiming the venom is the source of the Drider's longevity.

A Drider is composed of conventional matter, albeit unconventionally and incompatibly. They are natives of mundane space/time.

Humanoid Part (when operating on a human scale, eg. embroidery, throwing rocks, picking pockets)

STR 3d6 (10.5)     SIZ 3d6 (10.5)     DEX 3d6 (10.5)     Damage bonus nil

Arachnoid Part 

STR 3d6+6 (16.5)     CON 4d6+6 (20)     SIZ 4d6+12 (26)     INT 2d6+6 (13)     POW 2d6+6 (13)

DEX 3d6+6 (16.5)

Hit Points av. 28       Damage Bonus +2d6     Move 8 

Weapons:

• Human weapons, base % + Humanoid DEX, no db. No particular reason a Drider wouldn't use a bow-and-arrow, and no reason it wouldn't use a revolver either. 
• Grapple, base % + Humanoid STR & DEX. Against human scale targets only; if successful, will Bite on the next round. 
• Bite, automatic if Grappled. 1d4 hits + venom (the Drider can use some or all of its accumulated venom) and/or blood drain (d4 STR per round until dead or set free).
• Crush, 50%, damage = db. A Drider is big enough to crush multiple adult human-sized targets, one for every 12 whole SIZ points its spider body has. If able to jump or drop onto targets, it's 1 for every 10 SIZ.

Armour: 2 points on the humanoid part; 4 on the arachnoid. It can choose to wear armour, if available.

At 0 hp or lower, the Drider is dead (unless blown to pieces, disintegrated etc.), but will continue to fight on for three more rounds before finally coming to a stop, even if it’s on fire. It can only make crush attacks & is working on instinct only. The humanoid part flops and flails. 

The same applies if the humanoid part is decapitated, but not if major organs are destroyed. This is not common knowledge - the Drider may not know.

Skills: Dodge (DEXx2 + INT); Climb 90% (but can climb any surface & hang from ceilings); Jump (STR+DEX); Hide 60%; Sneak 75%; Sense Vibrations 40%; Cast Horrendous Shadow 75%.

Spells: Any average Drider will know 0-7 (d8-1). These are unlikely to be directly Mythos related, unless the Drider's state is due to Mythos intervention. 

If a Drider has POW 12+, it knows one spell per point over. 

Habitat: not too far from a plentiful supply of blood, with space for books and experimental equipment.

Sanity loss: 0/1 to see a Drider’s horrendous shadow; 1/1d6+1 to see a Drider; 1/d10 to realise that you have become one.