Triggering Strand

Abjuration Cantrip
Casting Time: 1 bonus action
Range: 30 feet
Components: V, S (a tiny bell and a piece of fine silver wire)
Duration: Concentration, up to 1 minute
Attack/Save: DEX Save

You create a nearly invisible, magical thread between two stationary objects or surfaces you can see within 30 feet of you, no more than 20 feet apart. The strand acts as a magical trip-wire, and creatures passing through it must succeed on a Dexterity saving throw against your spell save DC or fall prone and take 1d4 bludgeoning damage. On a successful save, they only fall prone.

The thread can be detected with a DC 15 Investigation check. If the thread is detected, it can be avoided, just like a normal trip-wire.

If you cast this cantrip alongside a 1st, 2nd, or 3rd level spell on your turn, the secondary spell is triggered when the wire is tripped. The secondary spell's effect is triggered from the midpoint of the wire, and only the effect of that spell occurs (e.g., a spell like Burning Hands or Sleep).