I ordered King Arthur: Collection[www.amazon.com] from Amazon. Their web page states that the bundle includes:

• King Arthur: The Role-playing Wargame
  
• King Arthur: The Druids DLC
  
• King Arthur: The Saxons DLC
  
• King Arthur: Knights and Vassals DLC
  
• King Arthur: Legendary Artifacts DLC

After activating the bundle's key on Steam and downloading it, I had difficulty determining if all of the above DLC were indeed present. There is no longer any web page in the Steam store for my particular version of King Arthur: Collection. It has been superceded, I believe, by a new bundle that includes more items. Within the Steam client, right-clicking on the King Arthur: Collection item and choosing Properties yields a Properties window with no DLC tab.

It's easy enough to see that the Druids and Saxons DLC are present in-game from the main menu by clicking on either the New Campaign or Load Campaign buttons. If these DLC are present, they you'll see a list of campaigns that includes The Saxon campaign and The Druids campaign. However, I could see no way to determine if either the Knights and Vassals or Legendary Artifacts DLC were present without playing through the game.

After some sifting through the game's install files, I've figured out how to verify the contents of the old King Arthur: Collection without launching the game.

\Steam\steamapps\common\king arthur collection\UI\DLC
will contain the file DLC_The_Saxons.dds

\Steam\steamapps\common\king arthur collection\UI\DLC
will contain the file DLC_The_Druids.dds

You can match the contents of the DLC listed on its Steam store page with files in the install directory.

\Steam\steamapps\common\king arthur collection\UI\UjBattleUI\Csapatok
will contain .dds files for each of the items below (for example Briton_archers_idle.dds)

New units become available after the completion of the "Lady of the Lake" objective:

• Briton Archers
  
• Briton Cavalry

New units become available are available after the conquest of either Norfolk or Norgales:

• Saxon Knight
  
• Welsh Pikemen

Two powerful Knights join the Round Table when the first Stronghold is built:

• Sir Alymere
  
• Sir Lucan

You can match the contents of the DLC listed on its Steam store page with files in the install directory.

\Steam\steamapps\common\king arthur collection\UI\ARTIFACTS
will contain .dds files for each of the items below (for example StaffofBlindHermit_idle.dds)

Three sets of ancient artifacts become available. Each set was designed for specific classes: Sages, Champions and Warlords. These artifacts are the rewards of nine new battle missions.

Sage Set

• Staff of the Blind Hermit
  
• Ring of the Blind Hermit
  
• Cloak of the Blind Hermit

Champion Set

• Sword of Sir Marrick
  
• Amulet of Sir Marrick
  
• Plate of Sir Marrick

Warlord Set

• Banner of Gorlois
  
• Ring of Gorlois
  
• Armour of Gorlois

I posted this because I couldn't find the information anywhere else. I contacted Paradox support to see if they could fix this and here was their reply:

I'm sorry but this issue can't be resolved. Instead of being the base game with the DLC separately installed, the collection was made in such a way that the DLC is all merged with the base game. Therefore Steam can't see that you already own the DLC; King Arthur: Collection is basically a different product from the version sold by Steam. We know that it isn't optimal but unfortunately we can't do anything about it except try to avoid the same issue in the future.

King Arthur II has no corresponding "Collection" as far as I know. There is only the base game and its single DLC named Dead Legions. The Dead Legions DLC, like DLC for most other games on Steam, doesn't have a separate listing in your library. To verify whether or not you have the Dead Legions DLC, you need to look at the details for the King Arthur II: The Role-Playing Wargame item in your library. Click on King Arthur II: The Role-Playing Wargame in your library in the Steam client and you should see details about the game appear in the right-hand pane. One of the sections of this pane is titled DLC and should list 4 items as follows:

• King Arthur 2 Dead Legions
  
• King Arthur 2 Forum Key
  
• King Arthur 2 Base_Release
  
• King Arthur II Base Game Game

It's a pretty messy listing. I don't know what the Forum Key is or why there are two base games listed, but the important thing to verify is that the Dead Legions item is present.

You can also see this same information in the Steam client if you right-click on King Arthur II: The Role-Playing Wargame in your library, choose Properties from the pop-up menu, and then click on the DLC tab.

Here is the order of the games:

• King Arthur 1 (with Knights & Vassals and Legendary Artifacts)
  
• King Arthur 1 The Saxons
  
• King Arthur 1 The Druids
  
• King Arthur: Fallen Champions
  
• King Arthur 2
  
• King Arthur 2 Dead Legions

The first three are included in the old King Arthur: Collection.

Hey, shout-out to Paradox! They messaged me to say that they used this post to help them solve a problem. As thanks, they gave me a free copy of Europa Universalis 4. Thanks Paradox!