The timed destruction of each dungeon floor, giving crawlers 17 days on the Hunting Grounds.
The sixth floor opens with a Time to Level Collapse counter reading 17 days, and the welcome announcement frames it the same way: crawlers have 17 days to find what secrets and mysteries lie hidden in the jungle.
Ajib shows what the mechanic costs those who cut it too fine — he had to jump through the portal just as the previous floor collapsed, and lost the bottom half of his left leg doing it. There are 2,344 exits leading to the seventh floor before the collapse arrives.
The countdown that forces crawlers downward, floor by floor.
Also appears in books you've already read — spoiler-safe to their endings: