There's 3 locations with temples, i don't remember area name, but one with temple surrounded by ice is gonna be first one you find, it will contain book that you need to equip on any of your characters, it wont bind tho so you can transfer it to any other character. Then it's island with succubus tail icon and there's a bunch on last island you unlock by chosing any victory path.
That's so sweet <3 Here's one more interesting secret to you, if you befriend foxraced girls and breed them together when at tier 5 of integration, they will have a chance to spawn with additional tail, you can breed up until 5th tier which will be 9-tailed fox as unic foxrace Trait. Sadly you can't transfer this trait to any non-foxrace girls with cross breading, but the have cool-looking model