The Eberron Campaign Setting introduces the religious adept
The adept's spell list is expanded a little by a variant adept presented in the Eberron Campaign Setting on—appropriately enough—Adepts:
A religious adept is identical to the adept NPC class presented in the Dungeon Master’s Guide (see page 107), with one important exception: The adept can select a single cleric domain. An adept does not gain bonus domain spells as a cleric does, but she adds the spells of her domain to her regular adept spell list and can prepare these domain spells in place of her usual spells.…
If a cleric takes a level in the adept class, he does not get to select a new domain, but he can add the spells from one of his current domains to his adept spell list. Likewise, an adept who takes cleric levels must use her adept domain as one of her two cleric domains. (256)
This allows some significant variation between otherwise generic adepts.
Other texts add a few spells
The following spells are added to the adept spell list by other sources:
- 2nd Level: bewildering substitution [illus] (Complete Champion 116), bewildering visions [illus] (Complete Champion 116), invoke the cerulean sign [evoc] (Lords of Madness 211).
- 4th Level: lesser spell turning [abjur] (Mintiper’s Chapbook Web column “Part 10: Chronicler's Compendium”).
- 5th Level: bewildering mischance [ench] (Complete Champion 116).
There are probably a few other extra spells added to the adept list floating around in other texts, too, but this is what I had at hand.
(Preface: The below answer was initially written before WOTC's acquisition of D&D Beyond on 2022-04-13. I assume the acquisition would not make my conclusion of 'treat the digital sourcebooks as official' less valid.)
I believe at least the digital versions of the sourcebooks themselves (currently listed here) should be treated as an official rules source roughly on par with the books.
Firstly, WOTC considers D&D Beyond to be an official digital toolset for the game (though, until the acquisition, D&D Beyond wasn't made by WOTC directly, but licensed through them):
This morning, Curse launched D&D Beyond—an official digital toolset for Dungeons & Dragons fifth edition.
(dnd.wizards.com, 2017-08-15)
Secondly, the books on D&D Beyond are considered to be 'digital sourcebooks', which, on their own site, are defined thusly:
A digital sourcebook is a completely digital version of one of the published books, such as the Player's Handbook or adventures like Curse of Strahd. You will get the book re-created in digital format, as well as unlock all of that book's content for use throughout the toolset - both for current tools and anything on the roadmap (such as encounter building/ combat tracking, etc.).
As they are considered a 'completely digital version of one of the published books', I'd say they're as official-source-worthy as said books.
In addition, BadEye (Adam Bradford, D&D Beyond product lead at Curse at the time of posting) also calls D&DB an official source, mentioning that their site is meant to be kept up to date with the rules as new changes come in:
Errata will be incorporated as it comes in. As an official source, it's important that we always stay current.
Ultimately, due to the above, I feel D&D Beyond's version of the sourcebooks should be considered as official as the books themselves. If there's a discrepancy between their content and the most recent errata/printed version of a book, then the book (plus relevant errata) is the 'most canonical', sure, but I don't think the potential for that kind of mismatch/data entry/out of date error would make the rules from the site overall considered 'unofficial'.
Disclaimer: The above only applies to content on D&DB from the official sourcebooks. D&DB also hosts various things that shouldn't be considered official, such as usermade homebrew, forum posts, and unofficial sources such as the Blood Hunter class from Critical Role/Matthew Mercer- these are usually kept separate in the site's navigation. The SRD & Basic Rules are also hosted there in a combined section, but even the WOTC-hosted PDF/printed versions of the SRD is considered unofficial:
The sword of sharpness deals an extra 14 slashing damage when you roll a 20 on its attack roll. The SRD incorrectly says otherwise. Note that the SRD is not an official rules source for D&D. #DnD
@JeremyECrawford, 9:54 PM - 4 Jan 2018
In addition, applicable Unearthed Arcana content was once hosted there (for the UAs released in the range of Jan 8, 2018 to the discontinuation of UA on D&DB around August 2021), which should be treated the same 'officially unofficial' way any other UA content is treated.
While the parts of D&D Beyond that are aggregates of data from the sourcebooks (such as the "Races" section, the "Monsters" section, etc.) contain reproductions of sourcebook data, they seem to be slightly more prone to data entry errors- I'd recommend using the digital sourcebooks' version in the case of finding a conflict between the two.
(On the topic of "what's considered an official sourcebook?", @nitsua60's longstanding question/answer "Where do I find the “official” rules for D&D 5e?" is worth a read as well.)
Best Answer
Blood Hunter is unofficial third-party content by Matt Mercer, but he and Critical Role have an advertising partnership with D&D Beyond.
Nothing makes the Blood Hunter class inherently special as compared to any other homebrew class published on DMsGuild or D&D Beyond. It's not any more official or AL-legal than any other homebrew class.
The only reason it has a special placement/presentation on D&D Beyond (DDB) is that Matt Mercer and Critical Role have an advertising partnership with DDB. Apparently, as part of that partnership, DDB has agreed to provide special listings for Mercer's homebrew Blood Hunter class and his Gunslinger fighter subclass. (The same items appear on DMsGuild: Blood Hunter,1 and Gunslinger.)
DDB's character creator also lists "Critical Role Content" separately from other "Homebrew Content", presumably for the same reason.
This changelog on the D&D Beyond forum discusses the partnership:
A Twitter user also asked about the partnership and the officialness of the Blood Hunter class:
In short: it's not official. It's basically "homebrew"/third-party content, just like anyone else's, except DDB has given it a special placement/designation because of their partnership with Critical Role.
This will not change with the release of the Explorer's Guide to Wildemount book in March 2020. Though it is the first official Wizards of the Coast book about Matt Mercer's setting of Exandria (and the continent of Wildemount within it), it doesn't affect the status of any other content by Mercer.
According to the product listing on critrole.com, the book will contain several new official character options, including 3 subclasses (the Echo Knight for fighters, and the Chronurgist and Graviturgist for wizards) and a number of new spells from Mercer's new dunamancy type of magic, among other new content. However, it will not reprint the content Mercer published on DMsGuild or in Green Ronin's Tal'Dorei Campaign Setting, or make that content any more official.
1 Mercer did release an updated/rebalanced version of his Blood Hunter class in January 2020, but the older version is still available as a separate listing on DMsGuild. He also mentions in this Twitter thread that the version on D&D Beyond will be updated as well.)