Nimbus Stratus-Serpent, also known as The Walking Fog, is a wingless, shifting amphiptere that thrives in high-humidity environments. Originating from the primordial oceans, these serpents are often seen in tropical regions but vanish in arid areas. They measure 50 to 70 feet long and consist of water vapor and ice crystals, displaying colors based on mood and time of day.

The serpents possess unique abilities, including generating sub-zero micro-hail and static discharges. Their diet consists of atmospheric moisture and static electricity, contributing to their role as rain-bringers in their ecosystem. They communicate through deep thunderous rumbles and are known for their peaceful nature but can become violent if threatened.

Lore describes them as dreams of the sky, respected by airship pilots but largely ignored by ground dwellers. Notable figures among them include Cirrocumulus the Endless. Nimbus Stratus-Serpent cannot be contained physically and has a long lifespan of up to 500 years. They are enemies of Steam Dragons, competing for territory and disrupting each other's atmospheric balance.