Demon Gates

Yesterday my roleplay group continued with an ongoing campaign in which we, valiant adventurers, have to travel across the land to close demon gates. It's your standard dnd campaign but it's one of my favourite campaigns.
I think this campaign is one of the few times were I, as a player, was actually stunned by certain events. In the near future I may plan to post the background of my character.
The thing is that my character is trying to figure out what is going on, because we have very little information about these gates, or the history to our world.

In the interest of me trying to figure it out, and not forget any of the clues that I've managed to find, I'm going to put down some of them here:

Long ago there were multiple gods in the world, now there's only one left.

At some point in time, a request was made to the wizard Ishtara, to help with the conflict against the orcs. But Ishtara betrayed them and they tried to kill her.

Ishtara at some point opened some, if not all of the demon gates.

Ishtara is the last of her race, her people once fought with the people of this world and against the gods of this world. Her race looks very similar to elfs, but with different ears.

Ishtara is now known as Lydia. She hasn't aged a bit for a few thousand of years. She is helping the party to, at the moment, close the demon gates. The armor she has given the party looks like the same armor that her people used to wear. Despite the fact that she is evil, none of her actions have confirmed it (other than her sometimes violent temper and the fact she rides a nightmare). She's extremely powerful but will not directly help the party unless we do something for her.

Demons are now using the demon gates to come into the world. And that's why we're closing them.

This is everything that I've been able to scrape together over an amount of sessions. My character has now had to make several promises that he would not continue his investigation into Ishtara.