Environment: Warm hills and underground
Guess so, it fits the original "tropical hills and ruins".
Organization: Solitary or x (1 plus x snakes
I'd say "Solitary or court (1 serpent lord plus 4-7 giant constrictor snakes plus 4-24 huge vipers)"
Challenge Rating 16 I guess, since they're pretty close to a 16th level cleric.
Advancement: 17–32 HD (Gargantuan); 33–48 HD (Colossal)?
We could follow the SRD Naga scheme of 150% (Size), 300% (Size+1), that would make them 17-24 HD (Gargantuan); 25-48 HD (Colossal)
Does their casting level advance with their Hit Dice?
Serpent lords are up to 50 feet long and weigh upwards of x pounds.
Well it's estimated a 30 foot anaconda would weigh about 500-550 pounds, so if we scale up to 50 feet that gives us around 2300-2500 pounds, which is way low for a Gargantuan creature.
Hmm, nagas start at 200 pounds, or 40% the starting weight of their Large size. If we use that as a basis and make them 40% of basic 16 tons of a Gargantuan creature that would make them 12000 pounds.
I'd trim that down to 10000 pounds, although its still four times the weight of an anaconda-proportioned snake that length.
Maybe all those fine cooked meals have made Serpent Lords seriously overweight?
