The black rain falls from the sky in unstoppable streams and turns the forest ground into a slippery obstacle landscape. The viscous liquid sticks to everything and etches itself unpleasantly into skin and clothing. Magical objects exposed to the rain gradually lose contact with the magical energy and become increasingly difficult to use. And it's no different with weapons, either. The greasy rain makes them slip from hands and makes it difficult to fight with them.
The path through the forest becomes more and more impassable. Small streams of black goo appear and cut off paths. Trees that lose their grip on the ground topple, or branches fall to the ground with a deafening crash. If you're not careful, you might sink into a mud hole and be swallowed by the earth. All that's left behind are a few air bubbles and the constant patter of rain.
Collecting. In the short timeframe of 5 (2d4) hours after the rain has somewhat settled, but the puddles of black goo haven't mixed with the dirt and mud too much, characters might try to collect some of the black goo for future use. Before then, the rain makes it impossible to identify clean puddles, and after that time the mix with mud has made the goo lose its potency.
To collect some black goo, a character must succeed on a DC 13 Wisdom (Perception) check, searching 16 (3d10) minutes to find a puddle of black goo that hasn't mixed with the mud too much. Taking an empty vial, the character then makes a DC 17 Dexterity (Sleight of Hand) check, to avoid both mixing the black goo with the mud too much, and prevent it from sticking to any items the character is wearing or carrying.
On a success, the character collects a single vial of black goo. On a failure, the character must search for a new puddle and one random item or piece of clothing is coated with black goo and must be cleaned off for 10 minutes with either holy water or magic.
This thick black liquid flows easily in the vial, but a thin layer always remains in any place it touched.
You can spend 10 minutes carefully coating a weapon, magical item, or arcane focus with the goo inside this vial, emptying it.
When a creature uses the coated item to make an attack or to cast a spell, attack rolls for that attack or spell are reduced by 1, and the spell save DC of a spell cast using the coated item are reduced by 1.
Cleaning the black goo off of the coated item requires 10 minutes of rigorous cleaning using holy water, or magic.
Magic Blockade. Whenever a creature takes an action, bonus action, or reaction that uses an item while exposed to the rain, the item gets slightly covered with black goo from the rain. Depending on the nature of the item, this has one of the following effects, which lasts until the item is properly cleaned:
Arcane Focus. For every time an arcane focus has been covered with black goo, spells cast using that focus have their to hit modifiers and spell save DCs reduced by the number of times the focus was covered.
Once an arcane focus is covered 10 times, it can't function as an arcane focus any longer.
Magic Item. For every time a magical item has been covered with black goo, spells and magical effects originating from it or using it have their to hit modifiers and spell save DCs reduced by the number of times the item was covered.
Once a magical item is covered 10 times, all of its magical effects are suppressed.
Weapon. Attacks made with a weapon covered with black goo cause a negative bonus to attack rolls made to attack with the weapon, equal to the number of times it was covered (maximum 10).
Burning Sensation. Any creature exposed to the rain for 10 minutes must make a DC 13 Constitution saving throw at the end of that time, taking 10 (4d4) acid damage on a failed save, or half as much damage on a successful one. On a 7 or less, the creature is blinded for 1 hour in addition to taking the damage.
Cleaning. For each time an item was covered with black goo, ten minutes of rigorous cleaning with clean water are required to remove one layer. The last layer requires holy water, or magic to be removed.
While the following effects continually happen, they are mainly relevant during combat, as they are otherwise relatively easy to avoid. Choose a random effect at initiative count 20 during combat.
Large Drop. A large drop of goo has collected on the branches of a tree above one creature. Any creature with a passive Perception of 15 or more, or that makes a successful DC 15 Wisdom (Perception) check notices the drop forming.
On initiative count 20 on the following round, any creature standing in the space above which the drop formed must make a DC 13 Dexterity saving throw.
On a failed save, the creature is covered in black goo. It takes 7 (3d4) acid damage and is blinded for 1 hour.
Goo Puddle. The ground around one random creature starts collecting black goo. Any creature with a passive Perception of 15 or more, or that makes a successful DC 15 Wisdom (Perception) check notices a puddle forming around the creature's feet, slowly causing it to sink into the ground.
Starting on initiative count 20 on the following round until the black rain ends, any creature that starts its turn within 5 feet of the space where the puddle started forming is restrained.
Any creature, including the restrained creature itself, can use an action to make a DC 16 Strength (Athletics) check, allowing a restrained creature within range to move to an unoccupied space next to the puddle on a success.
Slippery Slope. The ground in a 20-foot radius around a point centered on a random creature turns into a slope of running black goo. The area becomes difficult terrain.
On initiative count 20 of the following round, all creatures in the are must make a DC 17 Dexterity saving throw or take 7 (3d4) acid damage and fall prone and be restrained. All prone creatures in the area are moved 30 feet in the same direction. Each prone creature cannot breathe.
Any creature, including the restrained creature itself, can use an action to make a DC 16 Strength (Athletics) check, freeing a restrained creature within range on a success.
(we prefer ko-fi due to lower fees)