Now, you could do it as a straight-up fight, just by making a staged villain, probably a triple solo to make sure he has enough hit points to last, but that could quickly end in madness as everyone takes their turn to move and arrange and track their hit points and so on. Also, you'd need to add more area attacks or the things just going to get torn apart by sheer numbers.
So instead, I would probably set it up like an open ended skill challenge with an abstract map. The trick is to capture the two elements of a good raid: distinct roles, and a necessity to change up tactics.
The simplest way to do the "map" is to set up areas of engagement: Near, middle and far. Near means adjacent to the boss (where the main tank and melee damage dealers are going to be), far means at a distance (where most of the healers, buffers and ranged damage-dealers hang out) and middle is the space between, most occupied by skirmishers, off tanks, and specialized roles.
The boss, whatever he is, has a level. That level pretty much determines exactly what sort of threat he is, and also is the basis of most of his capabilities. His hit points are abstracted into the number of "Damage successes" it will take to take him down. I'm calling the ballpark on that his level (which I'll just call [L]) times 5, but this is totally unplaytested, so that might be off base, especially since it would probably be smart to scale him with the number of players. I'd endorse representing these with tokens, like poker chips, but it could just be that I'm nuts for tokens.
In a straight up fight, things go like this: On your turn you can move one "space" (or two spaces if you use any kind of movement power) or use a power (whether to attack or not). You might also do something else, but that's much more situational. Moving is hopefully self explanatory, but the actual fighting is something else.
When a player attempts to attack the boss, he can use any of his attacks, with the following limitation. Melee/close attacks require the character be near the boss, and ranged attacks (anything that would invite an attack of opportunity) need to be from the middle or far distance. If a character uses an ability to heal or buff an ally, that works normally.
An attack is always assumed to hit successfully, but most of the time the only thing that matters is the damage dealt - Raid Bosses are immune to all manner of special effects and statuses (except when they are not, see below), but benefits that help allies can still be triggered. If the damage dealt meets or exceeds a certain threshold (probably based off [L]), then it counts as a success, and the boss takes one point of "damage" - accruing damage successes is what ultimately takes the boss down. However, any failure gives the Boss one "Menace Point".
This matters a lot because, after everyone's taken a turn, the boss gets to go. He has a number of menace points equal to his level, plus any he's gained from player failures. He uses them to build his attacks - he should have a list of abilities that have menace point costs, but in the absence of that it works something like this.
For 1 menace point he can make an attack against a single target in the "close" area. It is an attack against AC, where the boss effectively rolls 15+[L] and does damage equal to the low normal damage on the table of the gods (aka page 42 of the DMG). For each additional menace point spent he can enhance the attack by doing one of the following:
- Affect an additional target
- Affect everyone in the area (costs 3 menace points)
- Target the middle distance instead
- Target the long distance instead (Costs 2 menace points)
- Increase damage one step to the right (low normal becomes Medium normal, high normal becomes low limited and so on, costs 2 points)
- Increase the effective "attack roll" by +2
- Change the attack to a different defense (Reflex, fortitude or Will)
This proceeds, round robin, until either all the PCs are dead or the monster has taken enough damage to go down. Simple as that.
Now, this is very basic, and very mechanistic (which is, arguably, very apt for a raid) with very little in the way of tactics. It would be intensely boring because most player will simply do the same thing every round. However, this lays down the baseline for the next step, adding in important things like aggro, roles, abilities and events. And that comes next.
1 - A staged villain is one who, when reduced to zero hit points, changes rather than dies. In most cases this is a physical transformation (like clay statues that have snakes burst out of their chests when they're beaten) but it's also a useful way to simulate the changes in tactics that are familiar to video game players. Normally, each "stage" has normal hit points for its level, though you can just as easily make one stage elite or solo (but I normally wouldn't - the point of this trick is to offset the long dull endgame of solo fights). Easy to budget for it too, as each stage is just treated as its own critter, which is a little bit kind to the players, but I think it comes out in the wash.
2 - As an optional rule, you might allow additional thresholds to speed things along, and to make strikers feel more valuable. So if a creature's threshold is 11 and a hit deals 23 points of damage, you might decide it removes 2 points of damage and so on. Alternately, that might just be situational, but that's something for tomorrow's post.
3 - I am tempted to have damage be measured in healing surges rather than hit points, but I need to think about the impact this has on healing, and whether it means tanks (sorry, defenders) end up insufficiently tough. If damage is in hit points, it's the one thing that can't just be handled with different colored poker chips, but the onus of bookkeeping is on players, so it is distributed. That said, tokens or cards definitely have certain logistical advantages, especially for the large combats. Similar thinking could be applied to damage (just say that at-wills, Encounters and Dailies do 1, 2 and 3 respectively) and while that simplifies things, it's actually a bad idea because it makes things REALLY boring for the player, and removes the tactical choice-making I hope to introduce next.
4 - This is, by the way, insanely abusable, which is why the DM shoud not actually use this on the fly unless he's willing to show some restraint. It's a decent yardstick for pricing boss powers though, and I'll use it later when I craft up some demo raid bosses.