Freya's Sigil and its Heroic mode equivalent are a portion of the third step in the Algalon the Observer attunement quest chain. Completing this quest requires the defeat of Freya in her Hard mode: leaving all three of her primary adds up while engaging Freya herself.
For more information on obtaining Freya's Sigil consult the Archivum Console.
Freya's Sigil is going to be one of the tougher ones, I reckon.
According to the Archivum, the essence of the Sigil is tied to her three elder servants, Brightleaf, Ironbranch and Stonebark. Should any of the elders be harmed, the Sigil's integrity will be compromised.
Sparing the elders, however, means that Freya will be more powerful when you face her.
What can I say, <name>. We're stuck between a rock and hard place.