Demon, Vloriak
Image © Paizo Publishing, accessed at Archives of Nethys here
[Rust monsters are an iconic D&D monster, but they are very explicitly a D&D monster. So as Pathfinder divested itself from the SRD, it couldn't use rust monsters anymore. The vloriak seems to be an intended replacement, which is pretty clever, because a monster that specifically screws with your PCs' gear is the sort of thing that players are going to really hate. So making them sapient and actively malicious means you can kill them without guilt! The biggest change I made to the vloriak mechanically was reducing the damage numbers to PF1e levels and giving it the standardized set of demon resistances. The biggest change in flavor text was explicitly discussing the similarities between vloriaks and rust monsters, because this is a fan project and I worry less about threading the needle of copyright law.
Help protect my creative works from collapsing into rust and ruin by supporting at the Creature Codex Patreon here]
Demon, Vloriak CR 5 CE Outsider (extraplanar) This stooped humanoid creature has red-orange chitin along its body and red-orange claws on all four of its gangly limbs. It has long curved horns growing above its four eyes, and an obscenely long tongue flicks in and out of its mouth.
The vloriaks are sometimes called rust demons or despoiler demons, both of which describe their activities well. They form from the sinful souls of saboteurs and terrorists, those who destroyed structures in life to cause death and despair. They were the servitor species of Xar-Azmak, demon lord of rust, who was slain by Dispater during a mad campaign to destroy the entire city of Dis. Since the Lord of Rust’s death, vloriak numbers have diminished but not disappeared. They are found throughout the Abyss, but are most common in Vlorus, Xar-Azmak’s abandoned domain.
Vloriaks enter combat as much to destroy items as to kill foes. Their tongues are longer than they are tall, coiling inside of the vroliak’s throat when not in use, and drip with acid that can supernaturally rust any kind of metal. Vroliaks delight in corroding armor and shields, as these items are often finely crafted and decorated with heraldry, holy symbols, and other finery. They can heal in combat by licking up fresh rust, and can spew shards of rusty metal and acid if facing multiple foes. Vloriaks are unlikely to fight to the death, fleeing from a losing combat in order to destroy more in the future.
The similarity between the insectile, rust-creating vloriaks and the rust monsters of the Material Plane is the subject of scholarly debate. Some sages believe that rust monsters are the descendants of vloriaks interbreeding with mundane vermin, whereas others believe that rust monsters are a magical creation made in homage to the vloriaks by some cult of Xar-Azmak long ago. Still a third opinion is that rust monsters are a product of convergent evolution, independent of vloriaks, and that the similarities are due to their diet of rusted metal. Vloriaks themselves are fairly stupid and not interested in discussions of monster studies.