Large TNT is an explosive that you will have to place and activate in the world to make it detonate and do damage on / destroy the surrounding. You can do so by dragging (a stack of) Large TNT into your quickbar (as with all explosives) and then put one of them on the ground with right-click. Then right-click directly on the bomb again to activate it - and quickly back away. Large TNT cannot be found in Treasure Chests nor be obtained from any creature currently (as of R28 in January 2016). To craft 4 bombs of Large TNT at once, you'll need (as of R28 in January 2016):