Cleon
Legend
Challenge 2 or higher is fine by me, just wanted to get an idea of what you're aiming for before we get too far into the conversion.
Here's a few more questions:
Firstly, the AD&D-ish White Dwarf and 4E Forgotten Realms Campaign Guide weres we're basing it on are both venomous, and in the original version that's a lethal save-or-die venom, so I'm thinking this version will do a fair bit of poison damage with its Bite which should justify a higher CR than a Wererat.
Secondly, is its Lycanthropy infectious? The 4E version is hereditary-only so a wereserpent bite doesn't transmit the curse of lycanthropy, it simply envenoms the target. The original entry says a victim must save vs. poison or die, it says nothing about them risking becoming a weresnake themselves. That leads me to think our conversion's Bite would not have the standard lycanthrope's "succeed on a DC ## Constitution saving throw or or be cursed with werebeastie lycanthropy" but require a save to resist being poisoned (poison effects to be determined).
Thirdly, what snake form does it assume? Your current rough draft has it turning into a "giant snake" which isn't precise enough. There are two giant snakes in the SRD, one Medium (the Giant Poisonous Snake) and one Huge (the Giant Constrictor Snake).
The White Dwarf Weresnake can summon giant poisonous snakes, which implies that it turns into one. That also conserves the creature's size category to Medium in its alternative forms. That'd make its wereform like so:
Shapechanger (Venomous). The weresnake can use its action to polymorph into a snake-humanoid hybrid or into a giant poisonous snake, or back into its true form, which is humanoid. Its statistics, other than its speed and AC, [?] are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.
If it isn't venomous but is strangles victims in its coils like the 3E Serpent Kingdom Wereserpent, it would be:
Shapechanger (Constrictor). The weresnake can use its action to polymorph into a snake-humanoid hybrid or into a constrictor snake, or back into its true form, which is humanoid. Its statistics, other than its speed and AC, [?] are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.
If you want to leave out the specifics of what type of snake it turns into as per the current rough draft, it should use "Medium snake" instead of "giant snake" like the shapechanging ability of a 5E yuan-ti or ti-khana. It'd use that if it turns into some kind of venomous constricting supersnake like a 4E Wereserpent. The stat block in the Forgotten Realms Campaign Guide says 4E Wereserpents "appear as a crush-grip constrictor" but I'm inclined to ignore that.
That should give it.
Shapechanger (Supersnake). The weresnake can use its action to polymorph into a snake-humanoid hybrid or into a Medium snake, or back into its true form, which is humanoid. Its statistics, other than its speed and AC, [?] are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.
Obviously what snake form it takes will be reflected in its abilities and powers. A giant poisonous snake is a lot more dexterous but less muscular than a constrictor snake, for example.
If it's a generic Medium snake that combines a giant poisonous snake and a constrictor snake then its physical ability scores might be the best of both or an average of the two.
