GM Created Side quest Question
(Ezeer, Finrich, or Pyrrhos don't read this).
So new DM running Tyranny of Dragons, and been having so far. 2nd campaign I've DMed. 6 sessions in, and PCs have cleared Chapter 2 and are now in the in-between time reporting back to town, escorting Leonsin back, etc. But during the Greenest section, my PCs became convinced that there must be a Spy in Greenest somewhere, thats how the guard was caught so off guard and how the attackers become aware of the adventurers to then set up the Mill ambush. Went what the heck, lets run with it.
So some background to set up the situation. Two of the 3 PCs are associated with certain...grey?....organizations. One is an assassin with the Fire Knives, the other an enforcer/guard/operative for the the Zhentarim. I set it up so that they each have a ranking member of their organization that was present at the start of the siege of Greenest, and hadn't gotten involved to avoid revealing themselves instead posing as just traveling merchants (the Fire Knives guy was looking to set up a safe house/expansion following the purge in Cormyr and in my mind is about the equivalent of a lvl 10-12 arcane trickster, and the Zhent is a about 12-14 wizard who reports directly to the Waterdeep Manshoon (only one in my version, as didn't know about others when started building my take on the story) who was there to meet a caravan carrying some of Manshoon's own goods and investigate what happened to some of their caravans). So the 2 PCs told their current "managers" about the spy, and the managers already knowing about each other officially revealed and were working together to investigate the spy. Set some distractions by having a Kobold that surrendered try and attack the Guard Medic saying the cult would find out he betrayed them (going to have the real spy reveal he used Suggestion on the Kobold to try and distract). But the two managers eventually figure out the spy is meeting their handlers, and when and where. They come to the PCs to ask if they would like to help "hunt a rat?"
This brings me to my question. The Spy is going to be Guard who was passing messages during the siege by attaching notes to crossbow bolts. His handler though, my plan is to have it be a Simluacrum of Rath Modar. Basically since he is involved more in managing the whole Rebel Thay movement and its interactions with the Cult, that he creates a few Simulacrums to aid in other places. This lets me introduce Rath a bit earlier since he plays a big role in the campaign overall, and lets me have powerful foil to the 2 powerful managers (PCs by comparison are lvl 4). So my plan is to have the Zhent Wizard and Fire Knife assassin focussed on fighting the simulacrum while they tell the PCs to wipe out the guards, which will be a group of DragonClaws i think (still doing the encounter math). But that leads to my questions below:
What stat block should I use for the Simulacrum? Rath Modar's since its supposed to be him? A lower level Mage or Archmage? Something else?
Going to have the managers capture the Simulacrum not knowing its a simulacrum to question. Going to have it have Otilluke's Frozen Sphere "poison pill" that it pops when they start questioning it. What happens to a Simulacrum when it dies? does it crumble? Turn back to reagents? Dissolve?
Any other suggestions thoughts? Have you done something similar?