How does the Shadowsong bow deviate from the attunement standards?

Question

Original Item

There is an adventure from the 3rd season of Dungeons and Dragons Expeditions called DDEX03-07 Herald of the Moon. It awards a very rare longbow as the main quest reward, called Shadowsong. Shadowsong is a special version of a more general magic item, called an Oathbow. Here is the description of Shadowsong, from the Rewards section of the adventure PDF:

Shadowsong (Oathbow)

Weapon (longbow), very rare (requires attunement by a ranger)

Each of the elven oathbows are possessed of mythical power and ancient legends. Shadowsong is hewn from a supple length of yew and features curious green metal tendrils snaking through its length. It glows dimly in the presence of humans. The description of this item can be found in the Dungeon Master's Guide.

The full description of an Oathbow isn't entirely relevant to this question, but it can easily be found on D&D Beyond or Roll20.

Retroactive Guidance

This bow is a bit odd, as it can only be used properly by rangers. This is not the most convenient attribute, but it also makes it a bit more interesting and special. However, Wizards seems to have decided that this is a problem, and issued guidance in the season 9 content catalog (5 years after the release of the adventure) saying that this item is inappropriate for Adventurer's League play, and should be replaced with a +2 long or shortbow instead.

This adventure features an item that is no longer acceptable under D&D Adventurers League guidelines, as it deviates from the attunement standards in the DMG. Instead of Shadowsong, this adventure instead unlocks a +2 weapon (longbow or shortbow). This +2 weapon has the sentinel property and glows dimly in the presence of humans.

One of my players noticed this after we had already run this adventure, and after some characters had been reworked to multiclass into ranger to allow them to use it. Although some players were happy enough with the change, others were rather frustrated, as it seems to be compulsory to make the switch.

My Question

However, the explanation for this guidance doesn't seem to make sense. How does this bow deviate from the attunement standards in the DMG? The Dungeon Master's Guide explicitly mentions that an item which requires attunement can have a class as a prerequisite, on page 136:

Some magic items require a creature to form a bond with them before their magical properties can be used. This bond is called attunement, and certain items have a prerequisite for it. If the prerequisite is a class, a creature must be a member of that class to attune to the item.

So what's the problem?

Answer 1

It differs by limiting attunement to Rangers

The Oathbow in the DMG has the property (requires attunement). That's the attunement standard for Oathbow from the DMG. This item, even though supposedly an Oathbow, has the property (requires attunement by a ranger). The two differ, and that is not allowed.

I think it is as simple as that.

Yes, some items can have attunement that limits them to certain classes, but there can be only one definition of the attunement requirements for any given item, not two different ones. Either the item is limited to a class or range of classes, or it is not.

What I mean by that is that if AL uses a named item from the DMG, then this item must match the item in the DMG, plus allowed minor cosmetic changes. Other items can be created, but they then need to be new, different items, not those in the DMG. They could have called this an "Elvish Hatebow", and typed out what it does (with the same functions as an Oathbow, but limited to Rangers), and it would then not have conflicted with the Oathbow. But you cannot refer to something that is not an Oathbow, as an Oathbow.

There are other items that are counterexamples to this explanation, for example, Lightbringer, from Lost Mine of Phandelver, which is a +1 weapon that also can be made to shed light and then deals d6 radiant to undead, and Dragonguard, a +1 armor that also grants advantage against breath weapons from dragons. What does this mean? Either that +n is considered such a simple enhancement, that these weapons and armors are not considered for this policy of protecting named items. Or that WotC missed to change these items. Or that attunement is of special importance. Or that my answer is just plain wrong. Or that just adding a feature is OK, but not changing an existing feature. I cannot tell which, unfortunately. I think it is likely that simple +n items are not considererd "named" items, such as a bag of holding, ring of invisibilty, or oathbow.

What are the allowed cosmetic changes?

The rules in the DMG, on p. 142 instruct you that you can customize magic items in minor ways, without really changing the item:

You can add distinctiveness to a magic item by thinking about its backstory. Who made the item? Is anything unusual about its construction? Why was it made, and how was it originally used? What minor magical quirks set it apart from other items of its kind? Answering these questions can help turn a generic magic item, such as a +1 longsword, into a more flavorful discovery.

The DMG provide tables of such minor properties and quirks on page 143. Among them is the Sentinel minor property:

Sentinel. Choose a kind of creature that is an enemy of the item's creator. This item glows faintly when such creatures are within 120 feet of it.

So you can have a longbow +2, that has the Sentinel property and glows faintly when humans are within 120 feet of it, and it is still a longbow +2. The feature that Shadowsong glows dimly in the presence of humans, can be implemented like this. The name of the specific item, "Shadowsong", the look, feel & backstory all can be likewise changed without creating a functionally different magic item.

However, changing the attunement requirements is not listed as one of the minor properties or quirks. So you cannot change the attunement, and still have the item be the same item. If you change the attunement on an Oathbow, it is no longer an Oathbow.

Answer 2

Meaning of the Content Catalog Guidance

I discussed this with some people on the official Dungeons & Dragons discord, in the #al-rules-discussion channel. They seemed to think that "the attunement standards in the DMG" referred not to the general attunement rules, but simply to the Oathbow's attunement property.

In other words, they made the claim that these sentences are synonymous:

This adventure features an item that is no longer acceptable under D&D Adventurers League guidelines, as it deviates from the attunement standards in the DMG.

and

This adventure features an item that is no longer acceptable under D&D Adventurers League guidelines, as its attunement property is different than the listed attunement property for the original item it modifies, as listed in the DMG.

I think that's a bit of a stretch, but apparently the issue was "fairly heavily discussed at the time", and this was the common understanding of the sentence.

This doesn't do anything to explain why it is a problem for the attunement property to be different, but it does explain why there doesn't seem to be anything in the DMG's attunement rules that would disallow such an item: they were not intending to claim that those rules were violated.

Reason for the Guidance

It has been theorized that Dungeons & Dragons Expeditions (DDEX) adventures have limitations placed upon the available magic items, and that these items should be items from the DMG which may be modified with the minor properties found in the list there (as described in Groody's answer), but that this limitation does not exist for hardcover adventures (such as the Lost Mine of Phandelver, which contains items like the Lightbringer mace).

If this is the case, then the Shadowsong would violate this guidance by modifying the Oathbow's attunement property, which is not one of the allowed minor properties in the DMG list.

There is some evidence to support this theory.

A survey of the magic items available in DDEX adventures (aided by this community-mainted Google Sheet) shows only items available in the DMG or another hardcover (such as Princes of the Apocalypse).

Additionally, although I don't think we have access to the rules used for writing DDEX adventures, the current rules for writing Dungeoncraft adventures explicitly state that, when selecting magic item rewards:

You may add 1 minor property or quirk to each non-consumable magic item (see the Dungeon Master’s Guide, page 143). Properties must come from the table in the Dungeon Master’s Guide.

All of the base magic items in the provided magic item tables are found in the Dungeon Master's Guide, Tasha's Cauldron of Everything, or Xanathar's Guide to Everything.