The danger of using an older or unofficial is that Warhammer 40,000 is a living game. Games Workshop frequently releases:
The is the official, comprehensive rulebook (often available as a PDF or digital format through the official Warhammer app) that provides the complete rules, datasheets, lore, and detachment abilities for the Grey Knights army in tabletop Warhammer 40,000. grey knights 40k codex pdf
Once per turn, you can remove up to 3 Grey Knights units from the battlefield and place them into Strategic Reserves. At the end of your opponent's turn, these units can be redeployed, allowing for constant, tactical repositioning. The danger of using an older or unofficial