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Let's say I have a brood of codex Hormagaunts who want to use their Leaping to assault 12" into a squad of Battle Sisters hiding behind some bushes. How does the difficult terrain roll work?
roll 3 dice, pick the highest and double it. (i think, i don't have the rulebook with me)
TzarNikolai is correct. (Y)
This is, in fact, rather clearly stated in the Codex rules for both Hormagaunts (Army List entry) and the Leaping rules: You roll Your normal (+ Fast&Deadly) amount of D6 for Difficult terrain movement, pick the highest as normal, and then double it since the creatures are Leaping.
"The meat things seek to destroy ourselves. They hunt us as we hunt them but they are weak and uncertain. The inferior flesh will be entirely destroyed, all biomatter consumed. Ourselves will fight to the last, all weak flesh must be devoured."