Starblight Crystal
All creatures within a (X) radius sphere must make a Constitution saving throw. On a failed save, a target takes 4d8 radiant damage, 4d8 necrotic damage, is blinded, and is considered poisoned until the target recuperates or receives magical healing. On a successful save, a target takes half damage and is neither blinded nor poisoned.
All creatures and objects within (2X) radius must also make a Dexterity saving throw, taking 4d8 force damage on a failed save, half on success. This effect should happen in the same round unless the targets are more than 1 mile away from ground zero and the blast is powerful enough to affect objects at that range.
A creature poisoned by this effect is vulnerable to necrotic damage for as long as it remains poisoned.
Irradiated Areas
Hazardous radiation is detectable using detect poison and disease.
Exposure Increment. This is what determines the intensity of the radiation, rendered as some length of time. Every time this length of time passes while a creature remains in an irradiated area, it must make a DC 15 Constitution saving throw or suffer the listed damage and poison effects. If a creature leaves the irradiated area before the first increment passes, it suffers no ill effects.
1st through 5th increments: Advantage on saving throw, 1d6 necrotic damage, poisoned 1d12+12 hours
6th through 10th increments: Normal saving throw, 3d8 necrotic damage, poisoned until recuperated, +1 level exhaustion
11th increment onward: Disadvantage on saving throw, 5d10 necrotic damage, poisoned until recuperated, +1 level exhaustion
Exposure increments are counted cumulatively until the creature takes a long rest.
A creature who fails three of these saving throws before taking a long rest also gains vulnerability to necrotic damage for as long as it remains poisoned.