I would consider two possibilities.
First, the Duskmother is a creature of power, but one confined to Spider Island. She has never appeared before in the lore or history and is largely an unknown. Prademar Adventurers have been conversing with creatures of immense power and intellect, including the Council of Dragons. for decades and this name has not surfaced. She may be the eldest of Spider Island or tied to the Island's energies itself, but nothing more. If annihilation of the Island is still the intent of the adventuring population, her corpse will be added to the pile.
Second, Duskmother may be another name (or perhaps an aspect) of Vdova, the Mother of Monsters. Vdova is a Prime Mover of Prademar, comparable in age and scope to the Council of Dragons. I would not be surprised if Vdova took Spider Island as her personal domain since it houses some of the most terrifying monsters in Prademar.
If this second option is the case, we would best prepare for parlay as we will have as much success destroying her as we would destroying Elders Black, Gold, and Red. She may very will allow us to depopulate Spider Island (she is immortal and will simply birth more horrors), but she will demand either sacrifice or compensation for eradicating a generation of her children.
Keeper of the Tainted Grove