The Space Marine bonus round rule is one of the many rules that can be applied during the Bonus Round episodes. It has a chance to spawn on floor 3 and further for every character, although it will not be offered if Cornelius is in the dungeon (since its effect would make Cornelius too strong).

Space Marine

Enemy countdowns reduce at double the normal rate.

When the rule activates, each dice the enemy uses in a piece of countdown equipment will reduce the countdown by double the value of the dice. For example, Rhino Beetle can now activate Beetle Headbutt, which has a countdown of 12, with just one 6.

This can cause a lot of trouble, especially if you're facing some strong enemies like the Rat King who will become very likely to get 6 Poison on you each turn, or Drake who will likely Blind and Poison you each turn with his large amount of dice. Most bosses will become much stronger with the exception of Scathach, so it's best to have equipment that locks or freezes dice if this rule is in effect.