Heart of Sadness

There is little necessary conversion work for the PCs' meeting with Myriana. If the party decides to fight her, or angers her in a way that causes her to attack, this is where you ought to consider a bit of restraint in pulling out the battlemat. She cannot be permanently destroyed, and would prefer not to kill any of the heroes herself, so I suggest running any such combat as a quick free-form encounter. If the PCs decide to destroy her, they ought to succeed in doing so. Though she is a creature of vengeance and capable of bending the swamp to her will, she isn't really a match for the adventurers if they work together to defeat her. However, if you feel it's appropriate given the specific circumstances of your game, you may get the chance to stop an overzealous PC in his tracks with her gaze. If this happens you can play it as a bargaining chip against them - it illustrates that Myriana has power, and that she might become a real threat to any nearby civilization if she isn't appeased.

Assuming the party eventually agrees to retrieve Lamatar's corpse, give them a major quest to reflect their task.