FREE ROAM AFTER FINISHING THE STORY: When you beat the game, it autosaves in the same slot you were using previously. After the end credits and cutscenes you'll return to the main menu. Select 'Continue' to return to the Skull Cave Base Camp in Paititi. Areas where there are no major collectibles (e.g., City of the Serpent, the landslide area north of the Mission of San Juan, etc.) will not be accessible, but most areas, including Cozumel, are reachable via the network of fast travel camps. You can also continue to build Lara's skills and explore any DLC tombs you haven't yet played.
You do not need all skills, weapon upgrades, or outfits, and the collectibles in Croft Manor and the various DLC levels do not count toward 100% completion.
There are 35 Artifact collections, for a grand total of 259 Artifacts. The 7 Artifacts in the 'Legend of the White Queen' (Croft Manor) collection are not required, so you actually only need 252 Artifacts. Finding these unlocks the 'Dr. Croft' Achievement/Trophy and contributes to 100% completion. 'Artifacts' include Documents, Relics, Murals, Treasure Chest rewards, and a few story-progression items, which are obtained automatically at specific points in the adventure. To see what you're missing, open the in-game map and page right to the Artifacts screen. You can then expand the section for each Story to see the individual Artifacts. (screenshot) Use this List of All Artifacts to clean up.
There are detailed walkthroughs for the 9 Challenge Tombs and 10 Crypts. (DLC tombs are not required.)
This page lists the 8 Side Missions, including when and where they occur, who the mission givers are, and links to the relevant sections of the walkthrough.
There 16 Challenges, but only 15 Challenges are required for 100%. The Bullseye Challenge in Croft Manor is not. All are covered in the walkthrough and Quick Guides. Challenge items, such as the ceramic whistles in the cave in Cozumel and the high-dive platforms in the Hidden City, are not shown on the in-game map but are indicated with colored dots on my annotated level maps, linked from each walkthrough and Quick Guide. To see which Challenges you've completed, open the in-game map and page right to the Side Missions screen. The challenges are listed below the missions. (screenshot)
Explorer Backpacks and Archivist Maps are not required for 100% completion, but they can help you locate other collectibles by revealing Documents, Relics, Crypts, and other points of interest on your in-game map. It is also not necessary to discover all Base Camps, but this can certainly make exploration easier.
If you're not sure which collectibles you're missing:
Open the in-game map (Tab on the keyboard, View button on XO controller, touchpad on PS controller). Use the direction keys or left stick to examine the various areas on the world map. Zoom in/out with the mouse wheel or trigger buttons. Total game completion is shown near the top left corner of the screen. As you hover over each area on the map, you'll see the percentage completion for that particular area. The key at the top left shows the number of each collectible you've found so far in that area. (screenshot) When you find a region where you are missing something, zoom in for more detail. You may be able to find the missing items on your map.
Undiscovered collectibles are bright white on the in-game map, while found items are grayed out. However, some items do not appear on the map at all until Lara gets close to them in the game or you find find a clue, such as an Explorer Backpack or Archivist Map, that reveals them.
In the previous example, we're missing a Treasure Chest and a Document in the Peruvian Jungle. Open the map, zoom in, and position the cursor over the document. Then press the button indicated on-screen for Toggle Waypoint. This marks the document with a blue circle. (screenshot) Now, when you close the map and activate Survival Instinct (Q on the keyboard or Right Stick button on controller), you can see the document outlined in blue. This can help you navigate to that area. (screenshots)
If necessary, refer to the walkthrough or Quick Collectibles Guide to help locate missing items. Each guide includes an annotated level map with collectibles numbered to correspond with the written walkthrough. If you compare my maps with your in-game map, you should be able to find what you're missing. Most walkthrough pages also include a section at the end on revisiting the area later in the game. These notes may be helpful, especially for levels where Lara's actions alter the environment or certain items can only be obtained using special gear.
If you need help finding anything, please feel free to ask.
*NOTE: The Prima Guide for Shadow of the Tomb Raider says survival caches are not required for 100%, but I can confirm that they are. When every region on the map registers as 100% complete, the Completionist achievement/trophy should unlock.
†UPDATE HISTORY: 8/25/21 - Page first posted online.
11/25/22 - Updated the artifact count. I had mistakenly forgot to include the Ix Chel and Chak Chel photo found in Cozumel. It's added to your inventory automatically during game progression, but even so, the count was off by one.
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