Underworld End and Rebirth.
The caves inhabitants all feel this time of year. Something within the deeps stirs. The blood quickens within them. A build up over the year for the oncoming final release. Predators become frenzied, conserved energy ready to be burst forward. Others prepare festivals and prayers, hoping for a positive and fruitful coming new year.
Even the caves themselves feel it. The chambers seem to flex with the coming energy renewal. Old creaking splintered doors spring back to life. Lichen recedes into the wall shrinking away. Fungi and fauna brilliantly bloom throughout.
The dungeons renews itself. It becomes alive, grows and spreads.