Cleon
Legend
"The Scarlet Death is an incorporeal undead being that inhabits locales that are frequently misty or foggy such as marshes or moors. Appearing as a roughly humanoid-shaped coalescence of fog with eyes like whitish beacons, it hunts and waylays travellers, using the weather to conceal its tracks."
Hmm…
A scarlet death is a vaporous undead that haunt marshes, moors and other locales that are naturally misty or foggy. It is so named because the normally whitish creature flushes red after draining blood. This undead resembles a small cloud with a vaguely humanoid top half with two arms that trail off like smoky tendrils and a head that's featureless apart from eyes like white lamps. The lower half is a misty mass floating over the ground. A scarlet death speaks in an eerie whisper but has no interest in communicating with the living.
Scarlet deaths are often called other names such as blood mist or crimson death, although these names can also refer to similar creatures the scarlet death is often confused for; like the undead vampiric mist, or the crimson death mist, a far more powerful monstrosity.
Secret Murders. Scarlet deaths are stealthy and intelligent killers motivated by their thirst for blood. A scarlet death takes great care to avoid discovery and prefers to waylay solitary travelers and hide their bloodless remains. The death's lair is a ghastly place full of decomposing corpses and treasure from its victims, these dens are always well hidden and the entrances are often underwater.
(given we are aligning it with the 3e monster...should we just call it Crimson Death...?)
I'd rather not use the same name for both 5E conversions to help distinguish them.
Since the original AD&D monster was some kind of living vapour I think it makes most sense to use Crimson Death for the high-CR monstrosity version and rename this undead version to something different, namely Scarlet Death.