Forum adverts like this one are shown to any user who is not logged in. Join us by filling out a tiny 3 field form and you will get your own, free, dakka user account which gives a good range of benefits to you: No adverts like this in the forums anymore. Times and dates in your local timezone. Full tracking of what you have read so you can skip to your first unread post, easily see what has changed since you last logged in, and easily see what is new at a glance. Email notifications for threads you want to watch closely. Being a part of the oldest wargaming community on the net. If you are already a member then feel free to login now.

Can Magnus use the spells from the codex instead of the index?
RAW, I would say no. See codex page 156 : Thousand sons ''can't use any rule and abilities'' listed there.
Is there a FAQ addressing this? I haven't found any.

Any interpretation have to confront this remark on page 156 which dictates how to use the section containing the new Hereticus discipline.

There is, lemme grab the link

https://www.warhammer-community.com/2017/08/13/daemons-and-thousand-sons-in-codex-chaos-space-marines-aug-13gw-homepage-post-2/

There ya go

I don't see anything in the linked FAQ addressing this, though I may be blind. Can you quote the relevant text?

Its not in the faQ, it's in the community page.

Secondly, if you’re a Thousand Sons player, you may have noticed that the Dark Hereticus discipline in the new codex is larger than the one in the Index. Don’t worry – the discipline isn’t being replaced and you won’t need to get Codex: Chaos Space Marines to keep using it. Similarly, if you have Codex: Chaos Space Marines, you’re welcome to use the expanded Dark Hereticus powers with your Thousand Sons Psykers. If you’re a Daemons player, this change also applies to Be’lakor.