Several large razors swing from pendulums across the bridge.
|The pendulums are easily avoided, but halfway through, darkness descends on the bridge.
A DC 10 Intelligence (Investigation) check detects the pattern in the razor swings. This grants advantage on the check to avoid them.
In order to cross the bridge, the target must make 6 consecutive DC 10 Dexterity (Acrobatics) checks. Failure causes 1d6 slashing damage from the razor.
However, halfway across, a 15 foot radius of darkness falls. This causes disadvantage on the checks to avoid the razors.
If the target stops moving, they take the 1d6 slashing damage every round at the start of their turn.
An aura of evocation can be detected on the bridge with detect magic or a successful DC 12 Intelligence (Arcana) check.
Dispel magic or a DC 14 Intelligence (Arcana) check dispels the darkness permanently.