The Spore Mother colonise the area under the well.
Eventually when it reaches a certain size, it begins producing eggs from it's cache of dDNA. (using all ettin breed slots)
It also grows hormone sensors, and if the local ettin population is below five, it will stimulate some of it's eggs to mature and hatch into new baby ettins.
To avoid overpopulation, unrequired eggs are reabsorbed before maturation.