Game progression are puzzles to unlock a mechanic or area in the game. Do note the keys are all interchangeable, so any keystone opens any door. Four of the gates cannot be opened before a quest is completed, but even then the keystone from said quest can open other doors.
Second option: trade it. (Note: do NOT have sex with her if you want to trade for it. It'll lock her into her sleep loop and you'll lose the ability to trade for the keystone.)
After you have sex with her, she asks for a futa/male Vulwarg with the Meaty trait or a female/futa Vulwarg with the Buxom trait. She asks your breeder's sex by default.
After you captured or bred a Vulwarg (they are in Lustwood) with the right trait, fulfill the request in the Special Request list found in the Quest Menu.
After you captured or bred a Thriae (they are in the Hivelands. They never spawn with hornball, breeding it from vulwarg tends to be the fastest way to get one.) with the right trait, fulfill the request in the Quest Menu.
Get past the Wooden gate leading the elf area. They are in the Esoteric Glade and you need to open Cockblock Gate to Goblin Cave in Amourous Hallows, then you can pull the lever in other side of the gate.
After you captured or bred an Elf with the right trait, fulfill the request from the quest menu.
In the Esoteric Glade go all the way to the back of the orc encampment (right hand side when facing the Elf/Slime part of the Esoteric Glade) and have sex with Chieftain Xena, then talk to her about the Keystone.
In the Moaning Crag, all the way at the back of the area you can find Pride on her throne admiring herself in the mirror. You can proposition her, at which point she gives you a challenge to create a Demon better than her.
She asks for a level 15 female Cambion with the following Traits: