when the deadlands are in a period of regrowth this means dagon's decidua is thickening again. when the deadlands are at their most disasterous and hostile to life to make room for new growth, dagon is menstruating. this change is heralded by his cervix plug, which is so thick so as to be impenetrable, coming out in a rubbery consistency.
when this happens, the inhabitants of the deadlands prepare. when his menstruation causes the deadlands to erupt into a series of natural disasters, most denizens avoid the incoming extinction event by seeking refuge in other realms, causing mischief and destruction in their wake. this can be likened to period stains (the ashlands of vvardenfell--where the land is dead, the volcano is active, and daedra are plenty--are a permanent period stain on morrowind's map).
dagon is known to manifest on nirn particularly at this time, and so is intrinsically bound to menstruation symbolism in his depictions. meridia is the one most equipped to deal with his stains, as her essence is that of bleaching acid rather than a purely destructive acid
