From publisher blurb:
Browsing through the Monster Manual, I realized that there are only three monsters in that book with a CR 12. Even with the tendency of having less high level monsters than lower level ones, I thought it would be nice to have some more variety - after all, two of those three available creatures are spellcasters.
I came up with this new monster, caled the Pustule Serpent. It is a venomous snake-like creature (albeit not really being reptilian, but that doesn’t make a lot of difference). One key mechanic of this adversary is that a failed attack against it may spawn of its young right into combat, by rupturing one of its poisonous gestation sacks. Poison damage and more danger ensues!
Other than the adult Pustule Serpent of CR 12 a younger version is also presented, having a CR 6. The key difference between them is that the older serpent has a poison spit attack and more damage resistances, making it a lot deadlier and also harder to defeat. Additionally the creature’s venom can be distilled into the highly dangerous blister poison, whose rules are also contained herein.