The last 15…
Fifteeen minutes can mean the world, particularly if it’s the last turn of a game. That last fifteen usually determines the outcome of the game, and that last 15 should be yours to control.
I say it over and over to people who play 40K: elect to go second whenever possible (exception being that you’re facing an army you’re sure you’ll table by turn 3 but if that’s you, shame on you for playing a leafblower, Goatboy.)
Why?

