"Neither steel nor stone could cause any damage, my Lord... It was simply too big, unlike any enemy we have ever seen."
Legion Captain Meron's last report to General Lorgar
"When the trees shuddered and the earth shook, we knew it was far too late to retreat."
Excerpt of General Lorgar's last report to Emperor Paulus, following the loss of his entire army of 10,000 men
On a location with numerous webs, and at least one big spider, there is a something inside a cocoon. It is humanoid in shape, still moving. If the heroes free it (not before they kill or drive away the spiders), they meet a ... zombie!
The poor zombie wandered the dungeon alone, and tried to kill a big living creature (= a spider). The spider used the usual treatment, even if this victim did not look tasty. The zombie can be easily killed as any other zombie. It got but several doses of spider-poison, so can be something worth if it is extracted. You can mention to a druid or ranger the fact the spider had no poison anymore.