Bright banana slug yellow and covered with a thick, jam-like, blood-red mucous, the crawling scream has the appearance of a chain of letters joined together, like one of those shitty Happy Birthday banners. Legs and arms of various letters dangle limply or probe around, seeking their own direction and sustenance but never escaping the original mass. It is lethargic until it detects the nearby breath of a creature, at which point it lunges quickly and slithers around its prey on a hit.
The scream's mucous is actually a psychotropic toxin, and it affects those who fail their save like a Fear spell, specifically a fear spell that induces horrible screaming. The crawler then squirms its 'head' into the mouth of its victim, slurping up all those yummy yells. This causes the forward section of the creature to grow in size. It will continue growing off the victim's screams until it is large and powerful enough to crush its victim to death via constriction. Then its own tiny mouth opens and the scream stretches, enveloping its victim like an anaconda and then quickly digesting them over the next hour.
There are also, of course, variants to this creature (Laffy Taffy blue or Horde Slime Pit green) that simply use the victim's screaming as an access point, wriggling inside them and devouring their corpses more slowly from the inside out.
The crawlers' new bodies segment grow in the shape of characters from a language spoken by the screamer. Their bodies over time become composed of dozens of different scripts, with their back ends composed of weird, forgotten, sometimes arcane symbols. Their flesh is littered with the last words of warlords and sorcerers; in fact, another name for them is Wizard's Last Words, for their propensity to devour shriekers, a warlock's usual early warning system, and slink away into the night....or into the wizard's tower.
They reproduce by budding, in the form of umlauts.