The Expanding Codex series fleshes out monsters from the Creature Codex, giving GMs ways to modify the existing monsters to surprise well-prepared players or to introduce monsters to a campaign.

Alternate Traits and Actions

The following changes allow GMs to alter an arcanaphage without modifying its challenge rating.

Diviniphage. A diviniphage feeds solely on divine magic but still interacts with all types of magic. It is immune to damage from divine spells. It has advantage on saving throws against other spells and magical effects. Its Hunger trait regains hp equal to three times the level of the spell and increases its Feed score by 2 each time it feeds in combat. Additionally, when it applies its Ingest Magic trait or uses its Voracious reaction, it can only feed on divine spells it ends or counters. Finally, when it dies and applies its Arcane Discharge trait, the force damage dealt by this trait is replaced with either radiant or necrotic damage.

The diviniphage loses the arcanaphage’s Magic Immunity trait, and other traits and reactions are modified as indicated above.

Magic Sight. The arcanaphage senses magic within 90 feet of it at will. This trait otherwise works like the detect magic spell but isn’t itself magical.

The arcanaphage’s blindsight is reduced to 30 ft.

Selective Eater. The arcanaphage is predominantly exposed to one school of magic (abjuration, conjuration, and so on), so it has adapted its feeding to better consume such magic. It is immune to spells and magical effects from its chosen school and has advantage on saving throws against other spells and magical effects. Its Hunger trait regains hp equal to four times the level of the spell and increases its Feed score by 2 for the selected school, but it regains hp equal to the level of the spell and increases its Feed score by 1/2 for all other schools. For purposes of its Arcane Discharge trait, ignore fractional values. Additionally, when its Ingest Magic trait applies, a creature maintaining concentration on a spell from the arcanaphage’s selected school has disadvantage on its Constitution saving throw. Finally, when the arcanaphage uses its Voracious reaction, it has advantage on its spellcasting ability check when countering a spell from its chosen school.

The arcanaphage loses its Magic Immunity trait, and other traits and reactions are modified as indicated above.

True Arcanaphage. A true arcanaphage feeds solely on arcane magic but still interacts with all types of magic. It is immune to damage from arcane spells. It has advantage on saving throws against other spells and magical effects. Its Hunger trait regains hp equal to three times the level of the spell and increases its Feed score by 2 each time it feeds in combat. Additionally, when it applies its Ingest Magic trait or uses its Voracious reaction, it can only feed on arcane spells it ends or counters. Finally, its Arcane Discharge trait deals 4 (1d8) force damage per Feed score to a creature that fails its Dexterity saving throw and deals 9 (2d8) force damage to a creature that starts its turn in the affected area.

The true arcanaphage loses the arcanaphage’s Magic Immunity trait, and other traits and reactions are modified as indicated above.

New Magic Items and Spells

The following magic item and spell are inspired by abilities possessed by an arcanaphage.

Arcanaphage Stone

Wondrous item, rare (requires attunement)

This smooth rock serves a similar function as rocks found in a bird’s gizzard and helps the arcanaphage digest magic. While you hold or wear the stone, you have advantage on saving throws against spells.

Additionally, you can use a reaction to counter a spell cast within 30 feet of you as if you cast counterspell. Unlike the spell, you must always make a spellcasting ability check (using Charisma as your spellcasting ability if you do not have one), no matter the spell level. If you successfully counter the spell, the stone increases its Absorption score by 1. Its Absorption score cannot exceed 8. Once you have used the stone to counter a spell, it can’t be used in this way until the next dawn.

Finally, you can use an action to throw the stone up to 60 feet away to unleash its stored magic. When it reaches its target, it explodes, and each creature within 5 feet per Absorption score must make a DC 14 Dexterity saving throw, taking 1d6 force damage per Absorption score on a failed save, or half as much damage on a successful one. This destroys the stone.

Devour Magic

5th-level abjuration (sorcerer, wizard)
Casting Time: 1 action
Range: Self
Components: V, S, M (a tentacle from an arcanaphage)
Duration: Concentration, up to 1 minute

After casting this spell, you can take a reaction when you are the target of or in the area of a spell or magical effect that allows a saving throw. When you take the reaction, you have advantage on your saving throw. If you succeed on your saving throw, the spell or magical effect has no effect on you, and you regain a number of hit points equal to the spell’s level.

Arcanaphage Adventure Hooks

• A wizard asks the PCs to capture a live arcanaphage specimen in a lead-lined jar she provides. She pays well for their trouble and ensures they are aware of the threat posed by the creature. She refuses to tell the PCs she plans to release the arcanaphage in a rival’s laboratory.
• A collective of arcanaphages has devoured all magic in the dimension they inhabit. The creatures wriggle their way, one per day, through a rift leading from their home dimension to a large city. The city’s leaders ask the PCs to close the rift before the arcanaphages can wreak havoc on the city.

If you have any requests for monsters from the Creature Codex for future installments, please let me know in the comments, and I’ll make sure to add them to the queue.

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