Fallen Knight

Thank you. Why can't I attack the Castle of Lightwood? Where can I find the Necromancer's Amulet and the Ring of Sinners?

It's too early for you to attack Lightwood. By the way, did you kill the gnome?
If you have collected all three artifacts, the quest for which appears after capturing Mythgard - the Cloak of Deadly Shadows, the Staff of the Underworld, and the Skull of Marquel - you can safely attack the undead legion near Mythgard, then go through the portal, and then figure it out yourself...

Added after 17 minutes
Oh, and one more thing - is Holyfire under the control of the Blue player?