Here's an example of one dynamic lair:
Lair: Mountain, 20%
Map Location: _______
An ancient Zaharan cistern has collapsed here, creating a sinkhole, 300' wide and 120' deep. The remnants of the fluted columns that once supported the cistern are visible, like jagged teeth rising up from the waterline, and small metal objects glitter below the water. The abandoned cistern is the lair of 4 Manticores (as per Monster Manual). 6,000gp in Zaharan coinage is scattered across the floor of the cistern, which is used as a lure by the manticores.
So, when in mountains, if the party encounters manticores randomly, there's a 20% chance they actually discovered a manticore lair in an ancient cistern. When discovered, I write down the hex number where it is to be found in case the party returns to the area. Sometimes the party stops to investigate and sometimes they don't. Each lair is unique and can only be discovered once, so for very common creatures (goblins, etc.) create 2-3 lairs for each.
From the party's point of view, they cannot tell the difference between the pre-placed static lairs and the random lairs. It just seems like wherever they go, there is a mix of wandering encounters, lairs, and dungeons to be discovered. If they actually were to look at the sandbox map, though, what they would see is lots of empty hexes with unusually high clusters of monster lairs that happen to be along the routes they've traveled. (For maximum effect, tie the dynamic encounters into your story web, too.)
Balancing the Challenge
In a few games, mostly science fiction, player characters are relatively static, but in most RPGs, characters advance from weak-kneed apprentices to mighty heroes and experts over the course of play. If that's the case, you need to take the reality of the leveling curve into account when creating your gazetteer.
The easiest way to handle it is to set your sandbox up as a borderlands environment because it gives you a built-in structure that explains the gradient of challenges the party faces. To build a borderlands, first put a string of border forts (or towns, space stations, etc.) running along one axis of the map, about 1/3 of the way in. To the rear of the border forts, put the main town/settlement. Beyond the border forts will be "the wilderness" or "the waste" or whatnot, where the majority of the dungeons and lairs will lie. Place these such that the deeper the players travel into the wilderness, the more dangerous it becomes. Put a few areas of higher-than-normal danger close to the border but in geographically isolated places; for instance, an evil fortress high up on a mountain, or a very deep underground river. Put your mega-dungeons such that one is close and pretty easy, one is a moderate distance away and relatively hard, and one is far, far away and murderously hard. Build your dynamic lairs and wandering encounters such that they are at a mid range of difficulty (in classic D&D, this is the 5th-9th level range).
The result of this structure is that early on in the campaign, the party hangs near the border forts. They can't risk the dangerous wandering encounters of the wilderness (which will be several levels higher than them) so they tend to travel from border fort to border fort, assisting each fort in clearing out whatever threats are nearby. Then when they reach a certain level of confidence, they begin to go into the wilderness, knowing they can handle any wandering encounters they run into. Again, they work from border fort to border fort, but this time in widening circles of exploration into the wilderness. As their power peaks, they will begin conducting forays deeper in the wilderness, far beyond the border forts, perhaps capturing or building strongholds to use as a staging point for deeper forays into harder challenges. The occasional high-level areas close to the border (like the castle on the mountain) serve as a reminder of the evil that lurks beyond, and also as a nice taste of what they can expect when they are ready to go deep.
If you follow these guidelines, you'll have your hands quite full - certainly giving me enough time to pen my next column, on techniques to transform your sandbox into a "world in motion" that lives and breathes. Happy gaming until then!
The following books, blogs, and settings are recommended to any student of sandbox gamesmastering.
Alexander Macris has been playing tabletop games since 1981. In addition to co-authoring the tabletop games Modern Spearhead and Blaze Across the Sands, his work has appeared in Interface, the Cyberpunk 2020 fanzine, and in RPGA AD&D 2nd Edition tournament modules. In addition to running two weekly campaigns, he is publisher of The Escapist and president and CEO of Themis Media. He sleeps on Sundays.