Here is the full list of all 22 Halo 5: Forge (Windows) achievements worth 1,000 gamerscore.
Start a Forge session with another player.
Create a map in Forge Mode with 5 or more vehicles.
Change the Skybox within a Forge Map.
Save your first Forge map.
Create 50 Forge Maps with 50 or more Props each.
Create a Forge Map containing an object scripted to move.
Modify the COLOR, MATTE/METALLIC, or MATERIALS properties of an object.
Create a map containing a Halo 2 BR, Halo 1 Magnum, and SPNKR Rocket Launcher.
Change the Reference Color of any category in Forge Mode.
Play a Custom Game by yourself on a map you created.
Win 25 Custom Games.
Complete 117 Custom Games on player created maps.
Complete a Custom Game with at least a 10 KDA.
Bookmark another player's map.
Complete the Forge Tutorial.
Be in a Fireteam of 4 or more.
Attach a screenshot to your Forge Map.
Win a 1v1 Custom Game on a map you authored.
Complete a 4+ player Custom Game on a map you created.
Complete a Custom Game with 7 other players on a user created map.
Get 7 headshot kills in a single Custom Game.
Create a Camera and Animation Location on a Forge Map.