I just thought of something. You could have the heroes take a breather (as in getting some rest before pressing onward to Dracula) and they are encountered by/ambushed by Carmilla in their sleep for the first time. Kinda like Alucard and the Succubus in SotN.
Why, she could appear specifically to Lenora, teasing her. I wanted Carmilla to oppose Lenora already, since Lenora is a descendant of Sypha, who had her family killed by Carmilla's machinations.
I have more to add to the idea, so please let me hear what you think:
Carmilla can be introduced as an apparition to Lenora. I
might try to work a "sleep" scene for her alone to incorporate this, but I'm leaning more towards a "meditation" scene where Lenora is introspective, trying to sense something with her mind (the characters are in a rush to destroy Dracula before the eclipse, so a "breather" is kinda out of question here xD). Carmilla appears in her mind, blocking her abilities and teasing Lenora to go after her.
During the story, I'll have an entire tower ruled by Carmilla, a chapel tower, where a scene re-enacting Simon's marriage from Haunted Castle's intro will take place. Of course, this scene is nothing more than pure theatrics on Carmilla's part trying to distract the hero(es). Instead of Dracula kidnapping the bride, however, it is Carmilla, who "kidnaps" Laura, who is playing the "bride" role.
Later, the heroes gains access to the upper parts of the tower, where Carmilla resides with Laura. However, nobody is allowed to enter her room but Lenora.
Upon meeting Carmilla, plenty of banter is given, where Carmilla states that she's been waiting centuries to drink from the blood running on Lenora's veins for it belongs to someone who escaped her in the past (a reference to Sypha). So here Lenora fights both Carmilla and Laura and defeats them, gaining access to whatever was it that Carmilla was blocking. Laura is destroyed, BUT it's still not the last of Carmilla, who "disappears" as if she had been killed, lamenting Laura's death and cursing Lenora.
The REAL battle happens much later, near the end of the story, when one of the heroes happens upon her blood-filled coffin hidden deep in the castle's bowels. She rises enraged from inside it and assumes her true form. I have a specific hero in mind for this battle - which prompts Carmilla to banter some more, this time about the personal info concerning Dracula I mentioned up there - but this time she is finished once and for all.
Also, I want to say that this pretty much confirms that both Olrox and Carmilla are making it into the story... Oh well, the plots are too good for me to simply let go of :|
Carmilla will be ruling one tower, and Olrox will be ruling another. Olrox's part is slightly smaller than Carmilla's (by what I have already imagined), but it's needed to set up his twist. Smaller doesn't mean "less important".