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Hollow Earth Magic: mechanical changes in MiniBx Core


I’m working on a hollow earth setting for MiniBX, and started thinking about some mechanical changes to fit. In others blog posts when I beginning to work with the setting, I have wrote about the setting magic flavor, how the flux of it was energy and how this energy is drowned by the environment and it depletes the very world, the posts here explain a bit how this was being structured: energy flux, as magic, hollow earth first idea, corrupted magic.

To synthetize all posts ideas: 

  • magic is draining users and the world itself vital energy, 

  • the ancient society in the depths knows and started to (and almost completely) leave/left  this world.

  • Monsters, strange tech were left behind.

  • The surface society is not aware about all of this, just some “sages” or apocalypse prophets.

The original Minibx magic rule does not take this in consideration, originally it reads: 

Archetypes like Wizard, Cleric, and Elf allow players to cast spells. At first level, Elf and Wizard PCs can record one spell in their tome of magic, while Clerics can memorize one miracle starting at second level. Each level attained thereafter, a spell casting PC records one new spell in their tome of magic, or memorizes one new miracle each day in the case of Clerics.

Spells typically are used to attack a target or create a special effect on a target. The caster rolls to hit or channel an effect on the target, much like an attack roll, adding +Magic instead of +Attack. The dice the caster uses to cast the spell is based on its Circle (see below). (GDN 4)

For Elves and Wizards, each spell written in their tome of magic must follow the instructions below and has an effect according to the classic spells from other games like B/X. The player creates the effect, but the GM has the final say defining it. Each unique spell or miracle can only be used once per day. 

As said, some chances are really needed, I will list them:

  • Magic users all of them have HD chains with d6s now, so wizards now have the same HD chain lvl progressions as elfs. 

  • The magic rule does not have circles anymore.

  • The +magic progression for spell users is going to be the same as +attack progression for fighters.

  • All magic costs a die in the HD chain.

  • The effect is free form, and as the original rule the player creates the effect, but the GM has the final say defining it. 

  • The magic can be used as much as any HD chain die is still available for the magic user.

  • The spell effect (now called spell die) is the die with the double side of the magic user's original HD die, for example if the user uses a d4 to cast magic the effect will be a d8, if using a d6 the effect will be a d12.

  • When it is not against roll effect, like enhancing armor, summoning a monster…the magic user HD die is automatically lost.

  • If the opponent uses a save, the HD chain die is automatically lost.

  • If the spell is effective, the HD chain die is maintained but an environmental depletion/impact may occur. 

  • If the spell fails, the HD chain die is lost.

  • The impact takes place when the rolled number in the spell die is >10, when this happens another roll with the spell die must be made and the effect will be at the impact table below. 

Number rolled



Some flowers or vegetation dries up and dies. 


A hole is formed in the ground, its ends uncertain.


A robotic voice tells the caster to stop, he loses consciousness for a day.


A functioning third eye arises in the caster.


The caster loses all the body hair for months.


A huge noise causes everyone around to be deafened for 1 day.


Water carried in or around the players directly becomes a pasty red liquid and obviously not drinkable.


All surrounding vegetal life turns to ash, vision is blurred for 15 minutes.


Gravity is reversed, everyone must hold on for 2 minutes.


Surrounding people including the caster must resist 18 points of damage.


The sky turns red for days, night and day are indistinguishable.


A gigantic flying object emerges from the subterranean veins.

Some spells and effects examples:

Magic Missile

Charm Monster/Person

Mage Armor

Summon Monster

Detect Magic


2magic missiles with 1d4 or 2 magic missiles with 1d6 (+magic) damage/attack.At cost of 1d4 or 1d6 HD chain dice respectively.

Charms a 1HD or 2 HD monster. At the cost of 1d4 or 1d6 HD respectively, more HD chain dice can be used in a single cast.

Gives a 1d6 or 1d12to + armor. At cost of 1d4 or 1d6 HD chain dice respectively. 

Summons a 1HD or 2 HD monster. At the cost of 1d4 or 1d6 HD respectively, more HD chain dice can be used in a single cast.

Against roll versus the spell/magic casted.

1d8 or 1d12 (+magic) area damage. At cost of 1d4 or 1d6 HD chain die respectively.

Having this in mind, if a wizard that learned fireball is  a lvl 2 PC, and wants to spend 1d6 of his hit dice chain (2d6/1d4). The attack roll will be a 1d12 +magic. The PC rolled 10 and had +4magic, the total attack was 14, if the fireball hits, the die is maintained, and since it is not 11 or 12 an environmental impact doesn’t occur (it is not necessary to roll on the impact table). If the fireball doesn't hit (successful defense roll or a save), the die is lost, the wizard now has 1d6/1d4 at they HD chain. 

Thanks so much for reading, this is a first draft, more to come and of course edits. You can check the game and all the amazing content that is already available here.


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