Group checks and the ambush

In “How D&D Next almost made knowledge count” and in “Is it noticed? How to run alertness,” I discussed the inevitable success that comes when a group rolls to gain one success. The reverse of this phenomenon appears when a group makes a check and just one failure can drag down the effort. The designers of d20 role-playing games mostly ignore these issues.

No situation highlights the problems of group checks more than the ambush. Using the simplest interpretation of the rules, everyone in a group setting an ambush must roll to hide, giving all a chance to doom the effort with a single bad roll. When the targets of the ambush arrive, every target gets a chance to spot that worst hider.

Based on real life, you might suppose that ambushes typically work. The group setting the ambush has the advantage of planning, preparation, and surprise. They just sit out of sight until their targets arrive. Unless someone sneezes or the targets have x-ray vision, the ambush works.

Based on the game, the word “ambush” describes a imaginary event that can never happen.

In “Is it noticed?” I suggested a fix. I advised assuming that the targets of an ambush take-10 to spot it. In effect, you set the DC for the ambushers’ hide check based on the targets’ lowest take-10 to spot. While this enables one creature to set an ambush, it still fails when a group prepares an ambush and everyone must roll to hide. My method only gives a group a chance of setting an ambush if the GM either (a) requires just a single hide check from the worst hider or (b) relies on 4E’s group check rules for the attempt to hide.

In a comment, Sr. Rojo suggested a method for handling ambushes that I like better.

How to run an ambush

To run an ambush, follow these two steps:

1. Allow the group setting the ambush to take-20 in their effort to hide.This reflects their time advantage, which lets them pick a good site and then arrange themselves for maximum concealment. When you set an ambush, you have time to work out the best hiding place you can muster.
2. When the targets reach the ambush site, ask them to roll to spot. The DC to spot the enemy equals 20 plus the ambushers’ worst hide bonus.Unless the ambushers stink at hiding, the DC to spot the ambush may be unattainable for some targets, and will present a challenge to the rest. Unlike most group spot checks, this check presents a reasonable chance of failure. Rather than assuming the targets of the ambush take 10 on their spot check, you can let them roll, and still give the ambushers a fair chance.

As with any spot check, you can limit the check to those characters keeping watch and in position to notice. If the party wants to an ambush a company, only the few soldiers on watch get a spot check, not all 100 enemies.

Take 20 and the rules

Some rules lawyers might argue that the hide attempt does not qualify for a take-20, because it carries a penalty for failure. I disagree. Unlike climbing or disabling a trap, the act of perfecting a hiding spot carries no penalty for failure. Your best hiding place may not be good enough, but that comes later.

Dungeons & Dragons fourth edition and fifth edition both lack a take-20 rule, so this method requires some latitude with the rules as written. In practice, if the players set an ambush and you tell them they automatically roll a 20 on their hide check, no one will gripe.

If the players walk into an ambush, you, as the game master, set the DC they must reach to spot the ambush. Even in a game without a take 20 rule, a DC equal to the ambushers’ worst hide bonus plus 20 makes a good target.

Next: Is it found? How to handle a search