Merlin and Morgana practiced speaking telepathically every night, from their own chambers. They started with simple mundane conversations until Morgana could sustain longer and more complex conversations. That's how they began talking for hours on end throughout the day, and well into the night after their usual magic practice. That's also how they began trying to cause trouble for one another.
It started during court. Morgana was positively bored but had steeled her expression to one of dispassionate attentiveness. Two farmers were in a dispute over a cow impregnated by the other farmer's bull, and Arthur was drifting off.
Look at Prince Prat, he's about to start snoring. He shouldn't have eaten the second helping of breakfast this morning. Or the third.
Morgana glanced at Arthur, whose head was resting on his left hand, elbow propped up on the armrest of the chair.
Or any other morning. You know I had to put another notch in his belt today? He's going to be as fat as this man's troubled cow…
Morgana had to fake a sneeze to cover the smile she couldn't stifle.
"Bless you, Milady," the boring farmer said, then continued to drone on about the cow.
We can settle this now, just cook the cow for Prince Prat's supper, you know that's what he's dreaming about over there anyway…
She nodded demurely to the farmer and held her handkerchief to her face to stifle her laughter.
Merlin, you're positively wicked. You're going to get me in trouble.
She looked at him, leaning against the back wall and he grinned at her, before slipping out the doors.
Later, she watched him from the tower as he was in the practice yard with Arthur and the other knights. Arthur handed Merlin a shield, so he could block the blows as Arthur struck at him with a wooden practice sword.
Merlin! Behind you! she pushed into his mind.
Merlin spun around in alarm just as Arthur whacked him with the wooden sword from behind, knocking him to the ground with a grunt.
"Pay attention, would you, Merlin? You're going to hurt yourself," Arthur rolled his eyes at the lump of his manservant face down in the grass, as the knights laughed.
That was for earlier at court, she noted with a smirk.
Duly noted, Milady. He groaned on the ground.
She laughed mischievously to herself as she walked back to her chambers. Until she heard another voice in her head.
Sister… it's time for you to join me, learn who you truly are.
Morgana frowned. I already know who I am. Morgana Pendragon.
You are a high priestess of the old religion. What little self-study you're doing will not suffice for the war ahead.
What war? I intend to prevent war, not wage it. My brother will return magic to Camelot when he is king.
He has no right to sit upon the throne of Camelot. The crown shall be yours, Sister.
I will not seek to rob Arthur of his birthright, nor Camelot of its rightful king. Instead, I will stay by his side as his princess, and eventually, as his sorceress.
You are naïve, child. You will see Arthur Pendragon is no different from his retched father. Would you see your blood burned at the stake?
Arthur will not harm you, Sister. He has made a vow to me that you will be protected. He is not your enemy.
And what of our blood? Those with magic? Has he vowed to protect them as well?
He will, in due course. I am sure of it.
We shall see, Sister. We shall see what sort of king Arthur Pendragon will be.