Oky doky, I'm putting my post in the collab doc (KCandK Scene - The Den of the Slitch) because I took a lot of liberties with everyone--PCs, NPCs, slitches, witches, undead, the whole shebang. Add, revise, cut what you see fit.
Admittedly, I have a difficult time getting into your baddies' heads, so I do suspect you two will likely want to add a little from them here and there. And I definitely can't guess what is happening inside Saerenwen's head, so inner diaglogues and priorites I leave to you to confirm/revise as you see fit.
Some notes:
Mosic's first spell was Beacon of Hope which gives advantage on saves, including death saves, and lets you regain the max healing possible when being healed. He followed that up with a Healing Word for Ch'dau, seeing as how he had taken the brunt of the bone spikes.
He also used his last channel to destroy undead when he got the chance.
Ch'dau was held with a Hold Person. Even with advantage on the initial casting, our big cat man failed . By the second time I rolled for him, I imagined Mosic had dropped Beacon of Hope, losing concentration after taking a few hits. Ch'dau still failed his second attempt... I'm sorry buddy, I swear I used my good dice! He did finally succeed, so by the end of the post he's free to do as he pleases
I expect everyone will be pretty roughed up at this point, and I believe all of your baddies, including Tecla, have some more tricks up their sleeves, so I'll leave that to you after I get your updates and post-post this bad boy.
Alrighty, have at it!