By the time anyone had noticed there was a problem, it was already well underway and impossible to stop, as these sort of problems tended to be. The rumble of trouble had barely run the full length of the gossip tracks by the time the windows were dark up through the owlery, choked shut even before the birds could make any panicked escape. It was gentle, methodical and horrible. The whole castle was, within the hour, wound up tightly in a spider's cocoon.
The spider had been spotted in various stages of the process. Well before anyone knew there was trouble, first of all, in the Transfigurations classroom, where Mr. Parker took it upon himself to test his skills on an innocent arachnid that made the grave mistake of wandering across his desk. In the moment, its body ballooned horribly, too heavy for its own legs, then it let out a dry fart of purple mist before seeming to slump over in exhaustion. Which was a riot all around, and embarrassing for Parker, but in the excitement Wicked took pity on the spider and took it back to the dorms to make sure it made a full recovery. As spiders do, it was thoroughly gone by the time it was next checked in on.
It was next spotted galloping outside near the lake, the size of a respectable dog terrifying the younger students who thought the worst thing they might spot was a giant squid tentacle. The hill remained mostly abandoned through the evening.
The first alarm was raised, very quietly, in Gryffindor tower, where a first year came wailing into the common room just past lights out in tears, insisting there was something horrible and hairy outside of the window. This account could not be verified, and they were labled a crybaby and this excitement had the whole tower still riled up by the time the head of house came rushing in and tried to organize the evacuation. It was too little too late.
Through doors and windows that had been caught open through the weaving of the web, the fibers could be felt, thick and very easy to get stuck on if one happened to be pushed against them. The crybaby was the first victim. His eventual quiet moaning of despair was dampened, much like most other sounds and light in Hogwarts, by layers of the spider's nest.