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The Light reveals the stairs going down clearly enough but fails to penetrate into the pillared room.

[because of distance or some other issue?]

The stairs go down 10' and the room stretches away for another 40'.
The Light is 20' bright and then 20'im dim.
The Light should therefore shed 10' bright and 20' dim illumination into the room.
But it doesn't.

Nix's witchsight can see the faint aura of the Light as it illuminates the stairs. there is no such aura entering the pillared room.

Something is keeping the light out, but it isnt casting a magic aura... antimagic?

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does this seem like antimagic or like magical darkness?

Anitmagic or possibly Wild Magic.

Nix quietly conveys the result of his inspection to the group

Opening her hand Alecia produces just enough light for her to see without tripping over, she pauses at times to note another mark on the paper before moving on.

Alecia updates her map and the group move into the pillared room.
Nothing else moves.

Sue close the distance on the item.

"If this is just your coin, I'm claiming it..."

=======================================
Cautiously and sword and shield willbe readied.

Saying nothing Alecia closes her map and stays close to a pillar.

Sue moves forwards and sees, behind a pillar, what appears to be another coin. The latter appears to be the one thrown in by Jim.
The other looks like a higher denomination coin. He also notices a single small dark rat which scurries off to the west.

The observant watchman notes that ther is something written on the diagonal southeast wall (although it will require illumination to read) and the northernmost pillar is encircled by the skeleton of a rather long snake. it remains deadly still in its spiral up the pillar, fanged jaws open as if waiting to pounce.

Nix stays near alecia, dagger ready.

[did his mage armor end?]

Ready for action Jim peers around. "huh, maybe it was just the rat I heard..." At Sue he continues on with "you can keep that one as long as you want to take point, or I can take it and continue." He smiles to show he is not upset "If you take my coins I will only have enough for a couple rooms"
If he notices the skeleton of the snake on the column - "Mister Sue, do you think that is really dead? He eases a dart free and poises it to throw at the skeleton"

Calming her breathing Alecia slides the map into her belt and reaches behind, finding a small tinderbox and a candle she lights it and holds the prayer candle at arms reach, just enough to see before her.