If you want to talk her back from the edge, you need to succeed on either a whopping DC 30 or a DC 21 persuasion check. If you can't do that, you either need to kill her, let her leave your party for good, or allow her to go through with spearing the Nightsong.
spoiler.
I experienced this is not always the case. My Shadowheart threw away the spear both when I encouraged her to do so successfully and when I just told her I had her back however she decides. Only in case of encouraging violence did she kill the Nightsong, so it might also depend on your previous interactions. I'm playing a good druid with Shadowheart as a LI.
There are multiple ways for Shadowheart to go during that scene, depending on how you have been interacting with her up to that point. I'm on a 'good' run, and I have maxed out her affinity. I didn't have to roll any dice. I only had to do something that is pretty obvious, if the choice is there. I guess if you haven't been going that route, convincing her not to kill said personage is quite hard.
Yeah I didn't say anything in this scene at all, I figured this was her business and let her get on with it, and she tossed the spear. I was kind of annoyed because I wanted to see if she got any cool Dark Justiciar powers, and she just up and decided to change her whole religion instead.
If she destroys the Nightsong, she gets a full set of unique Dark Justiciar armor (boots, gloves, helm, breastplate) and a Spear of Night upgrade. I think they have a bunch of powers that activate while you're in dim light. There's also a cutscene where Shar gives her orders to kill Ketheric Thorm.
It's cool but nothing game-breaking. I'm sure you get equivalent rewards for the opposite decision.
Unfortunately, as far as I know there is no single spoiler format that works across the fediverse. So no matter what people do here on lemmy, people accessing it via federation are out of luck.
Yeah, I made a post on the sync community about it a while back. Spoilers are going to be hard unless a common scheme can be agreed upon across the fediverse. People accessing the content from Kbin, Mastodon, etc, wont necessarily see things hidden either.
Yeah, unfortunately until the fediverse supports a single common scheme for spoilers there will always be this issue. Either that, or every single federated service on the fediverse has to support every other service's spoiler formatting, which is unrealistic.