The 2 ambush questions I saw on here are related to 4ed so I want to ask specifically for pathfinder(assuming its any different).
If I have players going through a dungeon and they are giving away their position either by sound or having a bunch of light sources and the monsters in the next room down the hallway are aware of the player's presence… Here is the question.
Do monsters have to be behind objects to get a surprise round? Im pretty sure if they are ranged and hiding behind a pillar, as an example, they do. But say the players are entering a square room from a hallway. If I have a monster on either side of the entrance, back a few squares, they are unperceived until the players enter at which time the creatures would want to strike. The tricky part here is these creatures have no cover to hide behind and they will be close enough to be revealed by light. Do they still get to do a stealth check against the players perception checks?
So in summary, If I have archers hiding behind 5 foot pillars, it seems clear that they should get to do a stealth check and then fire in a surprise round(please correct me if this is wrong, do they auto pass this roll since they are behind the pillars?) However if I have melee monsters waiting out of sight until the players actually enter the room, can these guys still do stealth to enter a surprise round.
A final bonus question on this same topic. Does only the very first player(s) to enter the room have a perception chance to act in the surprise round, or does the whole party?
Edit:
2 more questions have come up related to this in the comments that have not fully been answered:
Do the 2 monsters need to sit along the wall by the door with ready actions to charge somebody who enters in order to deny the player a perception check to detect?
What if the player claims that there characters should not be surprised because they are in a dungeon and are expecting a monster in every room. Thus they would enter each room ready for a fight. Does this make any impact?
Best Answer
Do monsters have to be behind objects to get a surprise round?
No. They have to be unobserved or otherwise have an surprise advantage over the characters.
http://paizo.com/prd/combat.html
Likewise it goes on to say
In your scenario you state:
It seems that you mean the monsters are unperceived by the party while the adventurers are being very observable, making lots of noise and carrying a light source prior to entering the room. If the party isn't being actively observant, these conditions should grant the monsters an automatic surprise round against the party, assuming the creatures have any chance of being in a position to attack the characters when the party enters.
Perception vs. Stealth should only come into play before entering the room (e.g. somewhere along the hallway).
If the monsters are observable before the party enters the room (the adventurers hear shuffling/snorting, the creatures give off a fetid stench, magical detection, active party awareness, etc.), this could warrant a check to stay hidden.
http://www.d20pfsrd.com/skills/stealth
This is where the Perception vs. Stealth check should be utilized. Unless the party is peeking around every corner with a mirror prior to entering (or has other ways of detecting the creatures), the advantage most likely goes to the prepped monsters for line of sight (thus being "unseen") -- if nothing else.
If you have a monster on either side of the door, whichever direction the first character looks will obscure the other monster. If you have a monster rushing a foe with a light, you can argue it's blinded by the light (negating surprise), but barring this, it is much easier for the creature to pinpoint the torchbearer than vice-verse. You also specified a square room, meaning that, side to side, directly left or right and closest to the door, only a length of perhaps 5' - 10' (one to two 5' squares) would be visible to the player(s) entering until both a light and head/half the full body of the first adventurer(s) were through the door.
Again, the creature(s) be should along the walls closest to the door. Standing away from the wall would likely move the creatures into a character's natural cone of vision (line of sight), negating its potential ability to hide (per illumination from a light sources).
If the creatures are clearly out of sight of the party and trying to remain out of sight until all the party members enter the room, you could use Perception vs. Stealth. However, it might be wise to ask yourself how the creatures know how many party members there are.
As I am not aware of any rules stating a Perception vs. Stealth is required to enter a surprise round and it is pretty clear Stealth assumes some sort of active giveaway, Perception vs. Stealth may not even be appropriate to utilize at all.
As for who gets to act, the characters behind the player(s) entering the room would almost certainly get no actions in the surprise round.
Assuming they cannot see or sense the creature(s) in waiting (Perception vs. Stealth if appropriate), then they have essentially no chance of acting. The characters entering through the doorway should be able to act in the surprise round, assuming they are not surprised (perceived the monsters prior to room entry).
EDIT
Yes. Pre-planning any attack never hurts. You should be aware of any special circumstances or restrictions on your proposed tactics ahead of time. For instance, if you follow the rules as written, Charging has special restrictions during a surprise round.
http://paizo.com/prd/combat.html
Suprise
Charging
Charging has other general restrictions including a minimum of 10' of movement, limiting attackers to a single melee attack (even if they are capable of multiple attacks) and having a clear path to an opponent. So it is an extremely good idea to evaluate your monsters' strategies ahead of time.
As a rule of thumb, I would feel comfortable in utilizing any attack that the creature would normally use, barring some special powers. A slime dropping on an opponent is not a special attack - it's how slimes hunt. Genuinely special attacks would not top my list of attacks to use except under exceptional circumstance. For example, a dragon, no matter how paranoid, would probably be unlikely to use a breath weapon immediately - they are almost certainly too smug about their general superiority. An exception could be made, however, if the dragon had recently been the target of another credible attack.
Regarding character awareness:
From a pure rules standpoint, it makes no difference whatsoever. Characters can always be surprised unless there is another rule stating that they cannot (such as from a magical item or effect). At the very least, every player has the possibility (in most cases) of failing a roll, especially if penalties are applied.
If you wish to concede that the party is always on their tiptoes and allow players a Perception check prior to entering every room with hidden opponents, this may appease some players, but not all. Inevitably, they will fail a roll at some point and be surprised.