Necromancer

If Sinthinia already knows that something is going on, chances are she's already got counter measures to solve it. Her overtures for an alliance, must be some what genuine to some degree otherwise she'd just let you go through with this plan and just have you all killed when you attempt it.

"Okay, Sinthinia I'm in." you say.

Sinthinia continues to rub against you and then looks into your eyes before kissing you.

"Mmmm, I can taste the truthfulness on your lips. A rare flavor around here. I'm glad you decided to join me; we're going to have so much fun together. Now, let me taste more of you. Something more…carnal."

A few hours later Sinthinia leaves your study (Which is now a mess) and remarks that she'll be expecting to see you soon after you meet with Sesisek.

A few days pass and you meet Sesisek in the forsaken area known as the Drozz Wasteland. The barbarian tribes here still hold sway, but you need not concern yourself with such minor threats, they know enough to stay away from two dark individuals such as yourselves, and the others…

The others in question are vampires. It would appear that Sesisek has been planning more than just "normal" betrayal. The vampires eye you warily.

"So, how did you swing a deal with the vampire clans?" you ask.
"Let's just say we all have similar interests. And that interest is seeing Velzix die and that bitch Sinthinia."
"Remember Sesisek, our primary target is Velzix! Your desire for revenge on other targets is secondary!" one the vampires exclaims.
"And I assure you that taking out Sinthinia is very necessary to our goals as well. She's head of internal security. Killing her is vital to our plan."
"I thought getting Mr. Necromancer here to mask us long enough to enter Veltrix's fortress was the plan?"
"He will be, but…"
"Hey hold on, what am I supposed to be doing now?" you interrupt while exaggerating your clueless act.
"You mean you haven't even informed him yet or what's going on?"
"I was going to do it now!" Sesisek hisses before turning to you. "Look isn't it true that you can use some sort of necromantic magic to hide our friends here? The reason why assassination attempts have been so unsuccessful before is thanks to all the magic detection security in the fortress."
"Oh. I see what you're getting at. Yeah, but I can't just do it myself. I'd still be giving off too much of a deathly aura that would probably arouse suspicion from Sinthinia's detection shit. I'd have to enter the fortress with either a bunch of zombies and skeletons, or I suppose I could just enter with a few of my abominations. Those would arouse less suspicion since I use them as bodyguards anyway. You vampires could travel with us invisibly, and you shouldn't be detected until you decide to go into a blood frenzy and start killing everyone."
"Ha ha! See! I told you he could do it."

You all "agree" to this plan and hammer out some other details before parting ways.

You head back to Aftermath, when your undead abomination bodyguards begin to sniff the air and you feel a presence stalking you, you stop and prepare yourself for an attack. Once again the vampire assassins become visible. Apparently they've been following you.

"You can't sneak up on a necromancer. I can sense your undead presence remember blood leech?"
"Is that what you still consider yourself? I thought you'd become a worshipper of demons rather than death, given how long you've been with them." One of the assassins says.
"No. I don't worship anything. I'm concerned with my own power; all of this is just a means to an end. And I have not given up the necromantic arts; you would be wise to remember that vampire."
"Good. Then it's not too late."
"What are you on about?"
"Look, this plan will be more successful if you fully cooperate with us. After we kill Velzix, Lord Rostov wants to invite you to join us. He feels that your connection and understanding of death along with your power would serve the Clans well. Among us, you would find a sort of peace. You would not have to deal with demons anymore."
"You must be joking! After I kill Velzix, I'm taking over! And anyone who opposes me is going to feel my wrath!" you say still keeping up the illusion that you're going through with this assassination.

The vampire assassin runs her fingers through her hair and pauses before speaking again.

"No you won't. And I'm not talking about a knife in your back by one of us either. I'm saying if you believe you'll be able to control an empire mixed with mortals, demons AND while controlling all your undead troops, then you're beyond delusional. Look at you. You're a wreck from all the infernal magic you've been taping into. Is it any wonder that demonologists don't live a long time? It's probably only due to your baffling plentiful necromantic energy that's kept you going this long. Even if you do manage to rein in the chaotic nature of the demons, and hold off the Grand Alliance, I doubt you'll live enough to enjoy any of it, let alone conquer anymore territory."
"I guess you bloodsuckers forgot that I can always become a lich."
"A lich? There hasn't been one of those since the time of the Great Lich Lord! The amount of death energy to become one must be huge! You're not there yet, you're not likely to get there and you're not pure enough anyway. You're tainted too much by hell. While you haven't directly sold your soul, you've thrown your lot in with them long enough to be damned. You will not be allowed to escape death on your own. BUT…becoming one of us will give you a chance. You will defeat death, and live lifetimes."

You think on the assassin's words. Much of what she says is true. You aren't in the best condition anymore. You're in pain a lot though you try to ignore it and not display your weakness. As far as being damned, you're not sure, but it's probably true. Your thoughts have focused less on death and more on demonic nightmares. The purity of what you once pursued HAS been tainted. You could almost kick yourself for allowing yourself to turn into what you are.

However, you wonder if the vampire is just telling you this to get you on their side. They must be worried about you if they want you to join this bad. Of course he could just be lying, but you don't think so. After being around so many liars you've come to see the signs, not to mention your own insights into the undead.