A huge tapestry of an ancient battlefield hangs on the wall. It appears to depict an army that is facing the viewer. Archers, spear men, battlefield wizards, and even siege weapons are in evidence.
Passing in front of the tapestry activates the trap, and the characters become targets of the soldiers depicted on it.
GM rolls d100:
01-35% 3d4 arrows fly out of the tapestry to attack the characters passing, targeting them randomly. The arrows are normal, but act as if they were fired by 6th level warriors.
36-50% 2d6 spears fly out, attacking characters randomly. The spears are normal, and act as if fired by 6th level warriors.
51-75% 3d6 sling bullets fly out, attacking characters randomly. The bullets are normal, and act as if slung from 6th level warriors.
76-90% 2d4 Magic Missile spells fly out, striking at characters randomly. Like the regular spell, these unerringly hit whichever characters are targeted.
91-99% 1d4 siege engine boulders fly out, targeting multiple characters at once. The boulders are normal, and do 4d6 HP damage to any characters struck (1-3 if they are standing in close proximity), acting as if launched by 6th level warriors.
00% Oh, bad luck! ALL of the above attacks occur at once!
The trap can be defeated by casting darkness in the area in front of the tapestry, so that the figures in it cannot see the characters passing by. It can also be defeated by destroying the tapestry with fire or other means.
Optional: Is that a dragon in the tapestry, as well...?