I think I have been wandering the Gael Serran for ages now and had all kinds of elite-roles but I am seeking the invitation to become a valkyrie, is there some item that I have to find?

Burn the Heart of the Lich in the black fire, then enter the Temple with any female character. The Heart of the Lich is in the pyramid. You need to jump onto the rising and falling column in the area where the champion keys are found to get to it.