Fighting a dragon in a field is a fight. Fighting the same dragon in its own cave is a different game, and the rules say so out loud. Every round, at initiative count 20, the cave does something. Stalactites fall. The floor opens. The air itself decides whose side it is on.
That is a lair action, and it is the most direct statement fifth edition makes about territory. This ground is not neutral. It belongs to something, and you are standing on it.
Players never get one.
The asymmetry nobody questions
Look at what a lair action actually communicates. It is not really about damage. Most of them are minor on the numbers. What they do is tell the table, every single round, that the encounter is happening somewhere that is on somebody else's side.
Fifth edition gives players a lot of things. It does not give them that. A player character can carry a portable hole, summon a wall, or shape stone, but they cannot make the room itself take a turn. The closest a party gets is a lair they have to defend, which is the same relationship inverted: the ground is still not theirs, they are just standing on it first.
For a game about playing monsters, that gap was the obvious place to start.
What official hag covens do, and why it is not enough
Hags already have a group mechanic in the rules. Put three of them within thirty feet of each other and they get access to some shared spells. That is it. It is passive, it applies whether anyone at the table notices, and it changes precisely nothing about how a round feels.
It is a footnote pretending to be a feature. Three of the most feared creatures in folklore stand in a circle and the mechanical consequence is a slightly longer spell list.
So coven magic in Roll Evil is built to be the opposite of a footnote.
What we built
Every hag has exactly one coven spell, tied to what she is. The Cinder Hag brings Burning Ash. The Sea Hag brings Maelstrom. The Annis Hag brings Rockslide. Six hags, six spells, and which ones your table has access to is decided by who showed up.
Three things make it a decision rather than a button.
It costs an ingredient. Not a magic item. A thing somebody had to go and get. Burning Ash consumes a Cold Cinder, an ember taken from a fire that killed somebody and never allowed to go out. Rockslide consumes bone dust from something that died angry. The ingredient is gone when you cast, and the Apothecary's Coven Speaker can conjure one, but what she conjures turns to dust in a day. You cannot stockpile your way out of the decision.
It is once per long rest, across the whole coven. Not once each. One. Four hags at the table means four options and one cast, so the question is never whether to use coven magic. It is whose, and when, and that argument happens out loud between players.
It counts the hags casting it. The damage scales on how many of you take part, so three hags together throw something none of them could throw alone. Standing together is mechanically load-bearing, which is the thing the official version only gestures at.
And then at level 8, it takes the ground
Every coven spell opens up further at character level 4 and again at 8. The level 8 tier is where the lair action arrives.
Four of the six become an action the battlefield takes, at initiative count 20, every round, for as long as the spell lasts. Not a one off. A standing feature of the fight, in the players' hands.
The Sea Hag's is the clearest. Maelstrom opens a vortex of dark water and lightning. At level 8 it becomes MONSOON: at initiative 20, every creature in the area makes a Strength saving throw or is pushed a distance that scales with the size of your coven, and anything that collides with a creature or an obstacle takes damage, and so does whatever it hit. A creature taking cover is unaffected.
Read that as a player and the tactical picture changes shape. You are not throwing damage at people. You are deciding, once a round, which direction the weather is going, and the enemy is now solving a movement problem you set. The Annis Hag's Rockslide does the same thing with the floor. The Green Hag's Acidic Swamp does it with the bog she made.
Design note
The lair action tier deliberately arrives at level 8 rather than level 1. A lair action changes what an encounter is, and handing that over at character creation would mean every fight for twenty levels is fought on the players' terms. Level 8 is late enough that the table has learned to fight without it and early enough that most campaigns get there.
What it cost to build
Two things, and both were worth knowing about before we shipped it.
The first is that a once-per-long-rest group resource makes a slow table slower. If four players each want to argue for their own coven spell, that is a real conversation happening in the middle of combat. The fix was not a rule, it was the play sheet: the coven pool is one tracker everybody can see, so the question is at least being argued over the same information.
The second is that scaling on coven size is a trap if you are not careful. Damage that counts your allies climbs fast, and the simulator caught it: at four hags the numbers ran ahead of where a spell of that tier should sit, and the dice counts came down before anyone played it. That is the whole reason the book has a simulator rather than a feeling.
Take the idea, not the book
If you run any game where the players eventually become the thing everyone else is afraid of, this is the mechanic worth stealing, and you can build it in an afternoon.
Give the party one shared, expensive, once-a-day effect that persists across rounds and does something to the battlefield rather than to hit points. Make it cost a component they have to go and find. Make it stronger when more of them commit to it. Then let it act at the top of the initiative order, every round, without spending anyone's turn.
The mechanical effect is modest. What it does to the table is not. The first time your players realise the room is working for them, they stop playing adventurers who happen to be winning, and start playing the thing in the cave.
Early Access
Coven magic is written up in full.
All six coven spells, their scaling and their costs are free to read on the site. Get on the early access list for the rest as it lands.