A line of sight question with a map (re: a monster hiding behind a wall)

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I'm planning an ambush for my Dark Sun PCs in an upcoming session and wanted some advice on how to make it happen properly.

After exploring a portion of UnderTyr, the PCs will discover a crack in the wall that usually separates the Golden City from the rest of Tyr (yes, it extends deep down into the earth). Using the "squeeze" action, they have to move through the crack. As the first PC emerges on the other side, I would like to have her attacked by a grell which has been hiding on the opposite side, up against the wall. If it goes well, she'll be immobilized and hamper other characters from emerging (though they can pass through her, they'll have to leave her space and thus draw an opp attack).

There are a couple of possible ways for me to position the grell so that the PCs do NOT see him crouched outside the crack.

First possibility:



A quick overview...the PCs begin in the light-blue area; the wall is the brown area (20 feet thick) with a jagged crack going through to the pale-grey room area (dark grey is solid stone). I want the PC attacked either in the final "square" of the squeeze, or in the first square as she emerges into the pale grey area.

Now, with the grell standing there, it cannot attack her in the crack (because the corner "fills the square"). But she CAN see it, correct? So my only option there is to have the grell make a Stealth check, use the corner as partial cover, and hope the PC's Perception check doesn't spot it. Not very satisfying.

Here's the second option:



With a step back, the grell can still use its reach to attack the PC when she emerges. Unfortunately, its Melee 1 beak attack won't be able to fire. Also...I think she'd still be able to see the grell, using the corner of her square, won't she? So there's no point.

What do you suggest would be the best placement for the grell? I want it to attack the PC completely from hiding, from 1 square away so it can use all attacks, and either in the last square of the squeeze or the first square of the pale grey room. 

Thanks!
There is always going to be the possibility that someone will spot it.

I would go with option 1, and pre-determined stealth roll.  If the Grell has reason to know that someone is coming in, then it can make the best of its situation, and adjust some debris, camoflague itself, whatever.  If it's your typical level 7 Grell, it has a Stealth of +17, so even assuming an average roll, DC 27 stealth is not bad.  +2 for favorable situations takes it to a 29.  Having time to devote to hiding, I could see making 3 stealth rolls and taking the best, or just determining that it rolled a 15, + other mods, giving it well over a 30 Stealth DC for the players to meet.  Which is probably beyond the scope of any Passive Pereptions, but still might be seen by someone rolling really well.

In that case, just go with it.  Sometimes PC/players just get lucky 
"Five million Cybermen, easy. One Doctor? NOW you're scared!" - Rose Tyler
Yeah, that's what I figured I'd be stuck doing; good point about the very high Stealth DC.

If I narrow the requirements further...let's say I wanted to make sure to attack the PC when she's in the crack space, not having emerged, with both beak and tentacle...what would be my best choice then? Having the grell shift into the square just outside the crack would be its one "surprise" action, without even attacking...the grell could Ready a shift, use that, roll for init/surprise and then use its one surprise action on an attack, but I guess it could not then bite and tentacle, just one of the two...
You can sortof handwave the very start of fights - as DM, you say when initiative starts.  If you want to give the monsters two actions worth of surprise, you can.  You just don't have to tell the players.  All they know is "BAMF!~  Grell attack!  Roll initiative."
D&D Next = D&D: Quantum Edition
I agree, and it's what I probably will do, but I was wondering if there was a bit more kosher way to do it I think the primary thing I want to happen is giving the players a panicked, claustrophobic feeling as they're trapped in a crack, squeezed in, their lead tank probably immobilized from grell poison, unable to really see what's going on or even target the grell with ranged attacks (due to the bend in the crack). I intend to get a lot of combat mileage out of a single monster in this case.
Here's a few extra things you might want to consider.  The first is to ask yourself if the Grell is waiting to ambush prey who might wander inside, if so it might be using the take 20 action if it has been waiting for 20 minutes or more.  This would give it 37 stealth if we went with the +17 mentionned earlier.  If it just got there or was chased inside it will still get at least a round or so to prepare itself and could make multiple stealth rolls to try to hide properly. 

Another would be to ask yourself if the party will be making active perception checks, if they aren't then using passive perception is completely ok.  If they are then consider the above.

Another would be to ask yourself if the Grell would be stealthed and have an attack readied.  Possibly the one that would immobilize, so that it could make it's attack from the first position you showed, and attack the last square of the small passage (diagonal to it).  This would give cover to the victim (because of the corner) but if the Grell was hidden by the stealth rolls and not detected it would have combat advantage to counter that.  This would give you desired effect and make it so that the Grell could only at first be attacked by the victim and force the other PC's to pass by and incur an opportunity attack by the Grell.

A surprise round would only allow for one action to the Grell, same for the readied action which it would take during the surprise round.  But if it rolls high enough on initiative it might very well go first during round 1 and allow for the attacks you mentionned.
"Non nobis Domine Sed nomini tuo da gloriam" "I wish for death not because I want to die, but because I seek the war eternal"

IMAGE(http://www.nodiatis.com/pub/19.jpg)

Good points one and all. Let me give you some more detail. In order to enter the crack, the party will have to make a lot of noise moving a pillar out of the way that's collapsed onto the other side, so during that period the grell (which is gliding around the other room) will take up a position beside the crack and wait.

I don't think the grell can attack diagonally, can it? If the corner "fills the square"? Or is it just that it can't move diagonally, but could attack with partial cover (-2)? Forgive me for such a basic hiccup
ok so I guess you need to decide how long it'll take for the PC's to get to where the Grell is waiting.

The corner fills the square means that creatures can't move diagonaly through it, they can however make attacks but the victim of the attack has cover which gives a -2 penalty to the attack roll.
"Non nobis Domine Sed nomini tuo da gloriam" "I wish for death not because I want to die, but because I seek the war eternal"

IMAGE(http://www.nodiatis.com/pub/19.jpg)

yeah thanks, I dunno why I brain farted on that...I've played it the way you describe countless times! I think I will use diagonal attacks, then, to ensure they get pinned down!
*cough* I think a readied attack and attack on surprise round then initative roll is enough - - It is important that the process is transparent to the players.

What is the grells perception - perhaps is the squishy stealthy char who is first through the crack - or a familiar (great scouts) - (also clairvouance potions)

 - - perhaps you ought to add a cloak  of stealth or similar as part of the treasure for the "adventure" and have the grell using it.


The entire premiss is that the "tank" is the first though   - perhaps the grell ought ot have backup plans if that is not the case.

The parties i have DMed and played in are not that predictable. 
It can be transparent, but it can also be whatever you want.

If you really want to have your grell move, claw, and bite on a surprise round, then let him do that.  If your players challenge, just say "It was an encounter power!"
D&D Next = D&D: Quantum Edition
****, thanks for mentioning the familiar possibility--our shardmind sorcerer just acquired a jankx familiar and I'm not used to adjudicating or planning for them. It's entirely possible he'll send the ol' sacrificial gerbil through first and I'll have to plan for that.

I feel like the tank will be first, but it could also be the rogue or even for some bizarre reason any of the others. Regardless of who comes through first, they're being subjected to this range of effects:


Traits


Human Shield


The grell gains a +2 bonus to AC while it has an enemy grabbed.


Standard Actions


 Tentacle Rake (poison)  At-Will


Attack: Melee 2 (one creature); +12 vs. AC


Hit: 2d8 + 6 damage, and the target is slowed and takes a -2 penalty to attack rolls (save ends both).


 Tentacle Grab  At-Will


Requirement: The grell must not be grabbing a creature.


Attack: Melee 2 (one creature); +10 vs. Fortitude


Hit: 2d8 + 6 damage, and the grell grabs the target (escape DC 16).


 Double Attack  At-Will


Effect: The grell uses tentacle grab and tentacle rake, or uses tentacle rake twice.


Minor Actions


 Venomous Bite (poison)  At-Will 1/round


Attack: Melee 1 (one creature grabbed by the grell); +12 vs. AC


Hit: 1d8 + 4 damage, and the target is stunned (save ends). The stun also ends if the grell is no longer grabbing the target.

(if the tank is the first, though...as a mul warden-gladiator, he'll probably use his incredible toughness to end the stun and will easily snap his way out of the grab. that'll be a nice heroic moment for him)
even if the Familiar goes first, there is no gaurantee that it will make a high enough perception to beat the Grell's Stealth
"Five million Cybermen, easy. One Doctor? NOW you're scared!" - Rose Tyler
if the familiar goes first, om nom nom nom.
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