Riposte!
[In combat anytime you have cover OR concealment from ALL enemies you may make a stealth check.]
"Where in the PHB does it say this? If you attempt a stealth attack against a target, what matters is the target's perception check, not any other monster's. A stealth check is made in conjunction with an action you are attempting to perform stealthily."
P.188
Cover or Concealment: Unless a creature is distracted,
you must have cover against or concealment
from the creature to make a Stealth check. You
have to maintain cover or concealment to remain
unnoticed. If a creature has unblocked line of sight
to you (that is, you lack any cover or concealment),
the creature automatically sees you (no Perception
check required).
A stealth check is made in conjunction with ANY action. FREE action is considered an action therefore by way of simple logic we can deduce that you may make a stealth check when you have cover/concealment. The "ALL creatures" part comes into play under the idea that if a kobold was creeping up behind your friend and he/she didn't notice it but you did, you would shout "Hey buddy watch out behind you" effectively making your friend "aware" of the sneaking enemy which in turn means it is no longer stealthed from your friend. As stated it is up to the DM's discretion to allow/disallow this depending on the situation. See the circumstance I had mentioned before.
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[As long as you maintain cover OR concealment from ALL enemies you remain stealthed. After you win a stealth check creatures are allowed to make a perception check on their turns as a standard action against your stealth check.]
"Again, where does it say this? Stealth is associated with actions. It is the action a player attempts stealthily that prompts a DC check, not simply "being stealthed", which is a 3e concept that doesn't apply to 4e. Stealth isn't modal, it's a method."
P.186
Perception: No action required—either you notice
something or you don’t. Your DM usually uses your
passive Perception check result. If you want to use the
skill actively, you need to take a STANDARD action or
spend 1 minute listening or searching, depending on
the task.
Opposed Check: Perception vs. Stealth when trying
to spot or hear a creature using Stealth. Your check
might be modified by distance or if you’re listening
through a door or a wall (see the table).
&
P.188
Stealth: Part of whatever action you are trying to
perform stealthily.
Opposed Check: Stealth vs. Perception (see the
table for modifiers to your check). If there are multiple
observers, your Stealth check is opposed by
each observer’s Perception check
[on]Success: You avoid notice, unheard and hidden from
view. If you later attack or shout, you’re no longer
hidden.
Right there. Again I state ALL enemies for reasons mentioned earlier and as ALWAYS it is up to your DM.
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[Allies grant you concealment ie: standing behind an friend would make you harder to see.]
The PHB indicates allies grant cover (physically blocking ranged attacks); not concealment.
Touché.
Regardless, allies still grant cover and cover is still grounds for a sleath check and my statement stands, though thank you for pointing out that mistypen discrepancy.
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"Can my dragonborn rogue stealth if he stands behind my halfling paladin in the middle of a battle? Do enemies just forget the dragonborn is there?"
Please exercise common sense in all responses. This situation would obviously be up to your DM's discretion. This is D&D after all, there are limitless situations one can find themselves in and it is up to the DM to allow or disallow certain aspects or bend certain rulings per situation. However unless the enemy creatures are truly dimwitted nothing 'forgets' you are there, they simply cannot get a clear look at you granting you a CHANCE to hide. Place a ferret on the floor and look away for one eighth of a SECOND and you will understand. You do not FORGET the ferret is around but I promise you unless you roll one HELL of a perception check you won't find it without a thorough search.
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"Can my rogue carry a large wooden block, "The Concealer," drop it at his feet in an adjacent square, and then claim he can stealth?"
Absolutely. If the block is large enough to grant cover (Ie; roughly the size of the character or however large/small the DM sees fit) and it blocks a part of all lines of sight to said character. Just as a character could throw a good sized sack of oil at an enemy and have someone else throw a torch at it, there are no specific RULES for that but you can certainly do it. It's called using your imagination, people seem to have something against it. I must have missed when it became a fault for trying to overcome obstacles creatively in D&D. Though I will say that getting to and walking through a dungeon carrying around a large wooden block is probably not the most effective mode of transportation. As always it comes down to the DM's discretion whether or not you make a "trip and crush your face off" check. Also why would your feet be in an adjacent square that sounds silly and painful. Also, also a more appropriate name would be "The mostly-portable wooden chunk of cover" as it is both more accurate and humorous.
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"Can my warlock stealth by using his shadow walk as concealment? Why don't the monsters pay attention to the misty, shadowy, swirly warlock-shaped object in the middle of the room? If this was intended for warlocks, why isn't stealth on the class skill list?"
Yes, you can absolutely hide in a heavily/lightly obscured square. The monsters DO notice the misty-warlock-shaped-area however as the warlock is concealed they cannot get a clear view of him/her. IE: the whole concept of concealment. If the Warlock makes the stealth check on that action it WILL incur a penalty of -5 for moving more then 2 squares, as per the requirement for shadow walk, on said stealth check against their PASSIVE PERCEPTION. Shadow walk was NOT intended to be used by warlocks to gain stealth advantages. It is obviously intended to grant warlocks a bonus to their defense by way of -2 to hit said warlock. SIDE OPINION: Warlocks (IMO, IMO disclaimer: IMO) are broken at the moment as they are not (yet) pact loyal. This however is an entirely different discussion not meant for this thread (read as: "Do not further discuss this matter in this thread").