There are of course millions of different types of bosses but one singular characteristic unifies them all: difficulty. All bosses should be difficult, or at least convey an illusion of difficulty. No one respects an enemy dropped in a single round. With this in mind, I've constructed a list of qualities I feel a boss should have, whether a one-night dungeon boss or a campaign-ending super villain, to maintain difficulty and cause your players to apply critical thinking. Feel free to disagree, these are merely my guidelines for myself.
1) Never allow a boss to be CCed for more than a round. CC stands for crowd control, it's a catch-all term used for any ability that prevents someone from acting or moving. If a boss can't threaten the players constantly, then he's not scary enough. If I feel my players enjoy CCing their enemies, I will often give the enemy some abilities that enable escaping from CC. Abilities like grease, freedom of movement, dimension door, or high Will saves. Even if an enemy can't cast these spells natively, they can still use magic items with their abilities built in. This rule also applies to things like trip and falling prone. A prone enemy takes attacks of opportunities while standing up. If players surround a tripped boss, they will destroy that boss very quickly.
2) Unless significantly higher level (and larger) than the players, never let a boss be by himself. One boss means one target. All attacks come straight for him, and before you know it, he's dead. A good boss has henchmen, whether they're low-level minions ripe for killing or big bruisers that act as bodyguards. They can slow down the players, allowing him to get out some of his best abilities, or they can be there to provide flanking. Whether they die fast or slow, the henchmen should always be doing something worthwhile. Rather than attacking, I often have my henchmen using crowd control, trip attacks, Will save spells like command, anything but simply beating on the players. In my opinion and experience, players are not afraid of damage, they are afraid of losing control. That's what a good henchman does, he helps his boss maintain control of the battle. Great henchmen force you to kill them first.
3) The environment is a weapon. Whether its traps, lava, mold, or acid, the setting a boss chooses to inhabit is part of his arsenal. At low levels, putting a ranger or wizard atop a tall cliff can make an otherwise bland encounter much more interesting. They can choose to range attack him, granting him cover, or scale the wall, leaving themselves open to attack. Put a deadly spore mold on the wall for an even deadlier encounter. Perhaps one that damages Strength, making climb checks even harder. Spumes emitting acid or lava are great and can be made random. Anything to make an encounter memorable. That's really the key here. I'll discuss more about environment in a later post.
5) Finally, give the boss an escape route. All bosses need a way out of a bad situation, and not every combatant will fight to the death. I try to avoid a forgone escape. If a player wants to chase down a fleeing enemy, that chase scene can be its own encounter. Or the second location they go to is the beginning of an even more epic fight. Escaped enemies can become reoccurring villains, and killed enemies can always be resurrected, perhaps to become even more powerful than ever before.
I feel that each of these elements creates a boss encounter that is much more interesting than a wizard in a room with an altar. A memorable boss and a great encounter will always stick in your player's mind, and will become one of the tales they tell for years to come. Thanks for reading, and stay tuned tomorrow for "C is for Cthulhu".