In Sly Flourish’s DM Deep Dive, Mike Shea explains how he adjusts encounter difficulty. “I don’t fudge dice, but I fudge everything else. A die roll is a die roll. That’s sacrosanct. But everything else around it is malleable.” This leads Mike to change things like the monsters’ hit points and damage numbers. The Dungeons & Dragons rules grant dungeon masters this freedom. The range of possibilities created by, say, a deathlock’s 8d8 hit points ranges from 8 to 64. DMs can choose the average of 36 hp or opt for another value.
I favor tweaking the numbers before the fight. This gains the minor benefit of letting me describe creatures as frailer or stouter than normal. I often dial up hit points for leaders, solo monsters, and other obvious targets for focused fire. In an encounter where a deathlock spellcaster leads a horde of lesser undead, the caster becomes an obvious target for focused fire and probably needs all 64 hp to live to cast a second spell. Some monsters need a damage boost to pose a threat. For example, gargoyles deal such feeble damage compared to their toughness that they turn fights into chores. My gargoyles may deal max damage rather than average damage.
Mainly, I refrain from changing the numbers during a fight. This helps me avoid the temptation to steer the game to suit my plans and expectations. Instead, the players’ actions and the dice guide the narrative. Sometimes during a battle I lower hit points to bring a battle to a quicker end, but by then the outcome is settled. Does this self-imposed restriction lead to more fun? Perhaps only for me. Players typically never learn whether adjustments came before or during combat.
In the Deep Dive, Mike Shea and his guest Ryan Servis mention a powerful way to adjust difficulty on the fly. Have the foes make better or worse tactical decisions—usually worse. Most often this means holding back a big attack or spell when using it could destroy the party. Sometimes it means changing targets instead of finishing a character, or focusing fire on an armored paladin or stout barbarian able take the blows.
Most DMs base some of a creature’s tactics on one roleplaying factor: the creature’s intelligence. They use smarter tactics with brainier monsters. DMs seldom dial up difficulty by playing low-intelligence creatures with cunning, but often this makes sense. Even beasts instinctively know to use their fighting traits in dangerous ways. Wolves gang up on the weak. Rats duck and cover. Tyrannosauruses bite before they swallow.
The most room to adjust difficulty comes from letting smart foes make weak tactical choices. Such poor choices can stem from roleplaying. In stories, villains frequently make bad decision, often because of the same character flaws that led them to evil. Their rage drives them to focus attacks on the wrong target. Their overconfidence leads them to save a devastating spell. Their sadism makes them leave a foe to suffer rather than dealing a killing blow. Their cowardice tempts them to run when they could have won. Their arrogance leads them to tell their henchmen to finish killing an apparently defeated party. A villain’s hubris can change a total-party kill into a second chance for victory.
In a battle scenes, bringing out such character flaws add a dimension to a villain while they explain the poor choices that spare the heroes. Still, you rarely need an explanation. Ryan Servis says, “The players never complain when the enemy makes a bad decision, but if you admitted to your players that you were fudging dice, they would all be upset.” Players rarely track all a monster’s abilities, so they seldom notice those fatal errors.