The Plan

Upon Arthur and Merlin' departure, Vortigern stood from the ruins of his fallen Kingdom, enraged by His defeat, Vortigern summoned his Wife, Morgana Le Fey, she was a former lady in waiting in Camelot but left of Her own accord when Arthur was crowned King. Morgana sided with Vortigern and his beliefs and became His wife. 

Vortigern waited patiently for Morgana to receive His message, to begin plotting His revenge against King Arthur and the Kingdom of Camelot.

Meanwhile in Camelot, 

Arthur and His Knights rejoiced in their victory against King Vortigern, but Merlin was worried, knowing what was to come. 

Arthur noticed the disconcerted look on Merlin' face, knowing something was wrong and so he walked to Him and said "walk with me."

So they walked through the Castle and while Arthur knew they had won a battle He knew something was amiss, and asked "you seem out of sorts, we have known each other for so long, please tell me what troubles you?"

Merlin looked down, knowing he could not reveal too much, but still duty bound to speak the truth. Merlin told Arthur His soul was troubled at what was to come. Arthur asked "what is to come?"

But Merlin said "I am not able to tell you everything the Lord has shown me, but know that God is in Control, and you should guard your heart, because it is the most deceitful thing, you cannot trust your feelings."

Arthur smiles unknowingly and says "But my friend, we have just won our greatest victory yet, what do we have to worry about? Just enjoy the moment."

Merlin exhales with a heavy heart looking Arthur in the eye. "Just know this isn't then end..."

Meanwhile Vortigern meets with His wife at their hidden Coven to discuss their plot against Camelot,

Vortigern being the violent one of the two only has death on his mind, but His wife Morgana, has better ideas with less violence and cost.