Fields of Ash, Mechanics

The Fields of Ash feed the source of spellcasting and most lifeforms in the world of Cinder, and is a phenomena known by few and understood by even less. In some places the Fields of Ash burn with a fierce intensity, and phenomena such as floating islands or portals to other planes of existence might appear. Eventually the embers die down and these areas may become barren wastelands due to the Fields of Ash lacking heat in that area, not allowing even a blade of grass to take root. There exist people, termed "Ashen Hosts" by some, who may possess an understanding of the Fields of Ash that few others have, and may manipulate it to empower their spell casting.

Each time the characters enter an area they have not visited before, and if the DM has not already determined an Ember Heat, then the DM or players can roll on the Ember Heat table to see the strength of the Fields of Ash in that area. An area usually refers to a single hex.

If the heat of the embers is Strong, Great, or Titanic, and a character has the Ashen Host Feat, then they may make an ability check when casting a spell to attempt to rake the coals of the Fields of Ash and conjure more energy for their spells.

The character uses their spellcasting ability modifier, and the DC of the check is equal to 10 + the level of the spell being cast. A succesful check rakes the embers of the Fields of Ash surrounding them and the spell is modified according to the amount of Ember Heat.

If the check fails but the result is within 4 of the DC, the spell is cast normally with no bonus or penalty.

If the check fails by 5 or more, the spell is cast normally but the spellcaster suffers backlash according to the Backlash Table.

Strong Effect Table

D10	Effect
1	If your spell attack misses or if the target makes a successful saving throw, you can choose a new target within 10 feet of the original target and within range. Make a new attack roll with disadvantage, or the new target makes a saving throw with advantage.
2	You can delay the effect of your spell for up to 5 rounds. Choose how many rounds you want the spell to delay at the moment it's cast. The spell takes effect at the start of your turn that many rounds later. All parameters regarding the spell's targeting, area, color, etc., must be set when the spell is cast, not when it takes effect.
3	If your spell deals acid, cold, fire, lightning, or thunder damage, you can change the damage to one of the other listed types.
4	If your spell affects multiple creatures, choose one. That creature has disadvantage on any saving throw it makes against the spell.
5	Double the range of your spell.
6	A creature that takes cold damage from your spell is restrained until the end of its next turn.
7	Creatures damaged by your spell are also deafened for 1 minute. A deafened creature makes a Constitution saving throw at the end of its turn to end the deafness effect.
8	Cast your spell without verbal components.
9	A creature damaged by your spell is also knocked prone unless it makes a successful Strength saving throw.
10	Roll twice on this table, or roll once on the Great Effect table.

Great Effect Table

D10	Effect
1	You can reroll a number of damage dice up to your spellcasting ability modifier (minimum of 1).
2	Treat your spell as if it were cast with a spell slot one level higher.
3	Creatures damaged by your spell are also poisoned for 1 minute. A poisoned creature makes a constitution saving throw at the end of its turn to end the poisoning effect.
4	You regain 1d6 hit points per level of the spell.
5	Your spell ignores damage resistance.
6	A creature that takes acid or fire damage from your spell takes an additional 3d6 damage of the same type at the start of its next turn.
7	Targets of your spell have disadvantage on their saving throws against it.
8	You can choose to exclude any creature you can see from the area of your spell.
9	If your spell has a range of self, you can cast it as a touch spell instead.
10	Roll twice on this table, or roll once on the Titanic Effect table.

Titanic Effect Table

D10	Effect
1	If your spell affects only one target, it instead affects up to six targets of your choice.
2	If your spell affects an area, double the area's size.
3	Creatures affected by your spell are incapacitated for 1 minute. An incapacitated creature makes a Wisdom saving throw at the end of its turn to end the incapacitation effect.
4	You gain a +5 bonus to your AC until the start of your next turn.
5	Casting your spell this turn uses only a bonus action; you still have an action to use.
6	Creatures immune to the damage of your spell take half damage instead.
7	Treat your spell as if it were cast with a spell slot two levels higher.
8	Concentration on your spell can’t be broken by damage.
9	A creature damaged by your spell is pushed 10 feet away from you and knocked prone. Its speed is reduced to 0 until it makes a successful Constitution saving throw at the start of its turn.
10	Choose one effect listed above.

Successfully raking the coals and casting the spell causes an adverse effect on the world around a character due to the Fields of Ash losing heat, according to the DM. Some examples include: 

Strong

• All the trees in a 30 foot radius have their leaves instantly wither and fall.
• The air in a 30 foot radius around them crackles with energy and seems to become unbearably hot and unbreathable.

Great

• Birds in the air around them fall to the ground dead, and seem to instantly being being consumed by maggots.
• The dungeon is filled with a horrible smell of rot, and though the stones seem untouched, worms wriggle out from between their cracks and fall dead at your feet.

Titanic

• All the living plant matter within a 300 foot radius instantly withers and dies.
• The falling rain instantly turns to ash in a 300 foot radius, forever cursing this portion of Cinder.

Failing to properly rake the coals causes the heat from the Fields of Ash to rebound against the attempting character, and they suffer an effect as described in the Backlash Table.

Backlash Table

D10	Effect
1	The spell produces only a harmless display of bright color and loud noise.
2	You are incapacitated until the end of your next turn.
3	You fall unconscious for 1 minute. You awake if you take damage or if another creature uses an action to shake you.
4	The next spell you cast fails, and the spell slot is wasted.
5	Targets have advantage on saving throws against your spells for 1 hour.
6	For 1 hour, you must make a DC 10 Constitution saving throw every time you cast a spell. If the saving throw fails, the spell fails and the spell slot is wasted.
7	You take 2d6 force damage per level of the spell that caused the backlash (cantrips cause 1d6 damage).
8	You lose one spell slot of your highest available level, and all charged magic items in your possession lose 1d4 charges.
9	Your spell fails, and you unleash a random burst of magic as if you'd used a wand of wonder.
10	You lose all your remaining spell slots, your connection to the Fields of Ash disappears for 24 hours, and all charged magic items in your possession fully recharge. You must make a successful DC 20 Charisma saving throw or fall unconscious; you awaken after 1 hour, if you take damage, or if another creature uses an action to shake you awake.

Ember Heat Table

D20	Ember Heat
1-11	Normal
12-16	Strong
17-19	Great
20	Titanic