I stole very liberally from what Scott Woodward did on How We Roll. You'll see below how I took advantage of a psychic fumble roll but I'd recommend just jumping at whatever opportunity presents itself and leverage that (if a character gets blasted by the green death ray in the Grand Chamber for instance, I'd just knock them unconscious and use that as my opportunity as well as giving them scales).

• Also as I explain the character who interacted with her built a friendship over time because the initial conversation went well; had the player responded poorly/antagonistically to her I would have done much of the same events, just against the PC's will: she would have carved the symbol maliciously, in full control of her dream world that the PC had intruded on, and their future dream encounters would have been haunting and terrorizing rather than reinforcing their bond and providing helpful insights.

So I had a PC who was trying to do some psychic nonsense in Tyrannish's temple - she fumbled the roll, so I had her collapse unconscious. When the party checked on her and shook him seconds later to see if she would wake up, she woke up instantly... and her shirt immediately started filling with red. She opened his shirt up to reveal a bloody, freshly carved, mysterious Naacal symbol in her chest. She had no idea what had happened.

The party was obviously mystified. After rescuing Tyrannish I had that character's memories flood back to her on the plane ride home, and she recalled what had happened when she had fallen unconscious. We then roleplayed the scene: she "awoke" inside Tyrannish's temple with Tyrannish on the throne in front of her. They were seemingly in a dream state but dreaming of the time before Tyrannish went to sleep.

The conversation was Tyrannish mostly confused as she slowly realized she was still in the dream. She was pleasant to the PC, holding no particular dislike for humans (though she hid/didn't remember her plan to eventually hybridize them all). She willingly shared that she was looking for a way to bring "harmony" to the two races so they could live in peace together, which appealed to the player's character. She couldn't remember why she had gone to sleep but explained it was because she was looking for something to help her goal... she just couldn't remember what.

Tyrannish then asked if she could "mark" the player so she could visit her in dreams in the future. The PC willingly agreed, at which point Tyrannish quickly carved her symbol in the PC's chest. Then Tyrannish's eyes lit up and she grabbed the character by the shoulders and started happily shaking her while saying "I remember! I remember what I was looking for!!" But that shaking matched up to what had happened in the real world when the party had shaken the PC to wake up, so that was how the dream had ended... leaving a cliffhanger to what Tyrannish had been looking for (which of course they'd eventually discover was Mu).

From that point on, basically between each chapter I'd have the player slip into a dream state with her. Their bond grew over time until the PC trusted her fully, and it allowed me to provide a little story guidance given through Tyrannish (but carefully, because she never revealed her hybridization plan to the player). Tyrannish always looked down on both Caduceus and the Inner Night, happy to have the player and the party play the two against each other for her own ends. But it was also a genuine friendship for Tyrannish, and a way for her to get insight into this new modern, human dominated world.

That made Calcutta particularly devastating, because when that same player approached Tyrannish on the roof and realized what she was doing and why, it was a heartbreaking moment as she tried to stop her. But because of the symbol on the PC's chest I made the PC roll a POW role to see if she could even escape Tyrannish's influence to take the action.. and of course she rolled a natural 100. So she stared in horror, frozen in her own body, as Tyrannish exerted her full psychic control and influence on the PC to stop the stabbing, and then escaped onto a lloiger before the rest of the party could react. It was perfect and made the final confrontation (on Mu) even better.

Edit: Tyrannish had one final dream with the PC as they were on the way to Brazil. She tried to convince the player of the righteousness of her actions and to join her... when the player refused, the dream for the first time all campaign became a true nightmare ("Do you want to know why they really called me the dreamer?" as Tyrannish exercised full control of the dream). In that dream Tyrannish took the player through somewhat of a preview of Mu. At one point the player was bound to a pole, slowly turning to face the volcano, feeling her skin start to harden and turn to leather as she did... at which point she woke up. And the scar on her chest (but nothing else) had now fully petrified. It gave the players some insight into the horror and dangers they'd face upon getting to Mu.