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[Sure. The blue circle on the wall.]

"Nice catch!" Galan pats his friends shoulder. "When we found the coin for the circle in the church, what else was in the box?"

[Not sure what you mean by box. No container was found for the coin or ring to go into, just a slot in the wall that went somewhere they can't access. Unless you mean where they found the coins in the pews. Those only contained coins except for the trapped one]

Faldair takes a piece of chalk and outlines the small area on the wall. He proceeds to scan the rest of the hall downstairs (pit, hall, stairs, wall, floor ceiling between the gas and the top of the pit). Nothing else is magical, he reports

"Well, I've been known to do quite well with poison. I'm willing to take a chance and walk through that gas cloud if we won't have any other tools to remove it." "I can try to hold my breath for as long as I can." "Since we don't know what that cloud is I'm willing the try and touch it to see if it burns."

"Any other ideas."

"Maybe experiment at the edge? That way we can pull you out...."

Van curious with the dot, will stand to the side a full arm length, and push it with the tip of his weapon.

Van climbs down to join his brothers and presses the indicated circle on the wall that Faldair outlined. The spot resists just like normal stone of a piece with the wall...

Zafron will put the tip of his 10 pole into the gas substance to see if it is acid.

Zafron waves his long wooden pole in the green gas. When he draws it back to inspect it, the surface seems a little damp (as if from a mist or gas condensing a little on it) but none the worse for wear

Van wants to ponder on the magic spot a bit.

Can he think of a past use of alteration magic on a stone wall? Some relevant experience or knowledge.

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Does this sound right for a description in 2E?

Alteration

Description: Spells of this school enable the caster to channel magical energies to cause direct and specific change in an existing object, creature, or condition. Alterations can affect a subject's form (polymorph other), weight (feather fall), abilities (strength), location (teleport without error), or even his physical well-being (death fog).

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yeah, basically alteration is anything that changes something from its previous form. hell, even "Knock" is alteration

"Wow! Haven't anyone told you that humanoids experiments are immoral?" Galan raises his eyebrows in surprise and looks parenting-ly at Zafron.
"Maybe you would like to taste it as well?!?"
After dramatic pause he adds, "I think we should focus on the magic dot. It may be the key to avoiding the gas. Does the riddle has anything reminding gas or dots? In the church we found the ring in a different room, so if we can't find something in here that relates to the spot it might be smart to climb up the pit and follow the tunnel we were in and continue the previous path until we find something relevant to here."
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(OOC: 1. We got the ring somewhere. I think it was in glass box that once was opened a snake or something got out, and we fought it. Am I that mistaken? I think there was three boxes (maybe wood silver and gold?) there with inscription below or something. So I asked if there was something else in these boxes/chests/...
2. Is the magic circle is hole or flat with the level of the stone nearby? If it is the latter it might conceal something and I suggest dispelling magic or breaking the wall around it, or searching for traps around it.)