Usually, when you’re about to spring a threat on the PCs, you have to take pains to find out what precautions the PCs are taking. That telegraphs to canny players that something’s about to happen. Here’s how to determine their precautions without ruining your surprise.

Present the party with an ambiguous encounter that they react to as if it’s dangerous (e.g., peasants with a wagon want to pass through a gate that the PCs are guarding). In fact, the first encounter is harmless, but it puts the players off guard and lets you see how the characters deal with potential threats. (If you’re lucky, they’ll also waste some spells on the harmless encounter.) Next, present a second, similar harmless encounter. Present it casually, and let the players say that they handle it "like they handled the first one" without going into detail. When the third or fourth ambiguous encounter happens, you can spring a combat on them without telegraphing your intent. You already know what the PCs’ precautions are because they determined their routine with the first encounter. With luck, you genuinely surprise the players.

Sometimes, two types of monsters work especially well together. If a combo is especially nasty, provide an XP bonus (like +10%) over and above the monsters’ combined XP awards.

Ogre + goblins: The ogre is the muscle and the main threat, but the goblins maneuver to give the ogre flanking bonuses, to keep the ogre from getting flanked, and to threaten the back-rank spellcasters.

Trolls + hell hounds: Fireballs don’t affect the hell hounds, so half of the spell’s potential goes to waste.

Harpy + monstrous scorpions: The harpy’s song doesn’t affect the nonintelligent scorpions, so they can fight normally while the group has to deal with the harpy’s song.

Jonathan Tweet, a senior designer at Wizards of the Coast, led the 3rd Edition D&D design team and authored the new Player’s Handbook. Since 1986, he has freelanced, self-published, and worked full time in the adventure game industry. His other design credits include Ars Magica, Over the Edge, Everway, and support material for AD&D, Magic: The Gathering, Netrunner, RuneQuest, and Talislanta.