Creeping Death (the Purple Worm)

9 and 30 Kingdoms recently posited on the wonder that is the Purple Worm.

Here's another possible wonderful thing:

Many sages posit that sometimes, ever so rarely, when the stars are right, the sun unflared, the tectonic plates shifted *just so*, an ancient purple worm finds itself a enormous underground geode, curls up, and metamorphoses into a Tarrasque - breaking out from the newly mineral-depleted geode to wreak havoc and lay eggs for it's limited, 6 month lifespan.