Warding Bolts

The Thunder Gates of Vorsthold have withstood regular assault for longer than their history records (the Vorstborn don’t care much for history). Armies of undead or Duergar or Drow or a panoply of Underdark denizens have thrown themselves in vain against these mighty walls.
Part of the challenge in defending the walls is seeing the attackers, given the deep darkness of the caverns - Dwarves can see in the dark of course, but their eyesight only reaches so far, not even the full range of a crossbow, for example. To solve this, long ago they discovered a kind of ooze that grew in certain caves that would give off a sharp flash of light in response to being hit as a defense mechanism - bright enough to scare away some insects or small beasts, but not enough to blind a humanoid. In a similar way to the Triton’s with Seadarias, they decided to put this ooze to use.
A sticky substance, when applied to a crossbow bolt and fired into an approaching enemy force it had an exponential effect. The initial impact would light the goo up and splash some of it around, illuminating the immediate area around it. If it hit someone and they kept moving - embedded in a shield perhaps - that target would remain lit up for a short while. If it hit the ground, the ooze would spread a little and anyone who stepped in it would leave glowing footprints whenever they took a step.
Many defenses of the Thunder Gates for years since have begun with a volley of these ‘Warding Bolts’, detonating into the opposing army with a burst of light, which the subsequent splashes and bit of ooze clinging to the underside of feet results in a rapidly spreading pulsating light across the battlefield. “Wards save lives,” is a common refrain drilled into new recruits - as their enemy gradually illuminates themselves with this sticky, reactive ooze they become increasingly easier targets to hit. Now, the floor of the run up to the Thunder Gates is so littered with the ooze that the Warding Bolts are hardly needed, and it’s known as the Lightning Pass, but some bolts are always kept in reserve in case a cunning enemy decides to clear out the stuff ahead of them.