In worlds where magic permeates the very earth, air and water, it is not surprising that some places become so infused with magical energy that they are overcome by it. These are the places where ley lines cross, where the membrane between dimensions is weak and where events so fateful have occurred the laws of nature themselves are twisted and tainted. These are areas where Arcane Auras are found.
An Arcane Aura is a zone of magical energy with a particular nature. It may be that the aura is caused by the very close link between the Prime Material Plane and the Elemental Plane of Fire, so the effects of the aura are based around heat and flame. Or it may be that the location was the site of a battle between sorcerers that was so fierce it broke the laws of normal magic and now wild magic rules there. Some rare Arcane Auras do not remain in place but move, following the lines of magical energy called ley lines or orbiting some other more “massive” center of magical power. No matter the specific nature of the arcane aura, a single fact is true for each one: the normal rules regarding the way magic works in the world are altered. This fact makes Arcane Auras highly sought after by scholars of magical study, as well as any who might benefit from a given aura’s effects.
Three factors define an Arcane Aura: its scale (how widespread the aura is in a geographical sense), its scope (how specific or general its effects are), and its power (roughly the equivalent of spell level, defining in game terms its Challenge Rating). Note that many Arcane Auras have additional descriptors, such as energy types or alignments, but these aspects are not required and vary widely between Arcane Auras. In addition, an Arcane Aura may itself change over time in any of the three factors, some regularly (for example, with the phases of the moon) or randomly by the whims of fate. Finally, most Arcane Auras are permanent (that is, player characters have no hope of changing or dispelling their effects) but some few can be modified with the use of strange rituals, powerful artifacts and other quest-worthy efforts.
General Rules: Each Arcane Aura is different in scale, scope and power as well as specific effects. Generally speaking, however, Arcane Auras operate by the following rules: the effect of the Arcane Aura occurs on any subject that enters the area affected by the aura. If saving throws are called for, they are made immediately upon entering the area. If a saving throw is called for every turn, it is rolled when and if the character or creature starts its turn with the affected area. All saving throws are against a DC of 10 + the Aura’s Power; the ability score of the save is called for in the text. Unless otherwise noted, the effects of the Arcane Aura leave the affected creature or character as soon as they leave or are taken out of the area of effect. If th effect of an Arcane Aura mimics or closely resembles the effect of a given spell, treat it as that spell and/or school of magic in regards to resistances, immunities, vulnerabilities and other special rules (i.e. if an Arcane Aura puts creatures to sleep, it does not affect elves).
An Arcane Aura is intended to be a unique effect based on a unique location that injects something of the weird or strange into the adventure and setting. The origin of every Arcane Aura need not be given to the player characters, or even necessarily developed by the Dungeon Master. These magical worlds have thousands, perhaps even millions, of years of lost history embedded within them, dominated by races and civilization incomprehensible to those that scour their ruins for gold and glory. The DM should allow his imagination to run free with Arcane Auras (indeed, with all aspects of setting and adventure design) and trust that the players will be suitably awed, terrified and/or impressed that the questions of “how?” and “why?” are moot, or at least satisfying in their unknowability. Certainly it is worthwhile to use an Arcane Aura as a plot point or a way to build the world in a knowable way, but the DM should be careful not to shackle themselves too much to the explainable.
With all that out of the way, here are some examples of Arcane Auras for use by the DM:
Ley Junction: Ley lines are the arteries that carry the power of magic through the world. Certainly, magic flows freely outside these ley lines, but its potent force is concentrated in the ley lines. Those with knowledge and means can discover the location of ley lines and sometimes build their towers and laboratories along them. True power is found where ley lines cross and meet, however. These ley junctions are rare, as most ley lines runs nearly parallel, but sometimes powerful magics can cause lines to bend and thousands of miles away they cross. In the most rare of instances, perhaps only at one location per world, three, four or even five ley lines cross at a single point. At these places, magical energy seethes and flows like water.
Scale: A ley line junction creates an empowered magic zone in an area 10^Power Level feet in diameter, centered on the point of junction or crossing.
Scope: Ley Junctions affect all magic within the area of effect.
Power: The power level of the ley junction is equal to the number of ley lines that cross. All ley junctions, by definition, consist of at least two crossing ley lines. To determine additional crossing ley lines at the junction, roll a d10 10. On a 1, the number of ley lines at the junction increases by 1 and the die is rolled again. For example, if the die is rolled and comes up 1, then rolled again resulting in another 1, followed by a 5, there are 4 ley lines crossing at the junction and the power level is 4.
Effect: Raw magical energy pulses at a ley junction, there for the taking of any arcane caster. The DC of all saving throws against spells cast within the radius of the ley junction is increased by the Power level of the junction. In addition, the Power level adds to any total damage or other effect roll (add it in just once, not per die). Finally, an arcane caster in the area gains a free spell slot equal in level to the Power of the junction while within. This slot is always available, but comes with a danger: any time a caster uses the slot gained from the ley line and rolls a 1 on an attack roll or a target rolls a 20 on a saving throw, the caster over channels magical energy from the ley junction and takes 1d10 arcane damage per power level and uses up all remaining spell slots. The caster may still use cantrips, if any, but gains no benefit from the ley junction. These effects end after a long rest.
Elemental Node: There are places in the world where the membrane between the Prime Material plane and the Elemental planes is thin. Magical energy of the elemental type leaks through the membrane and a small part of the Prime becomes very much like the elemental plane and elemental magic is empowered. Those arcanists that specialize in elemental magic often build bases of operation within arcane auras of this sort, as do creatures tied to the element in question (either of the Prime or the Elemental plane itself).
Scale: An Elemental Node has a radius equal to 100 feet times the Power level of the node.
Scope: The magical effects of the node only function for a single element (earth, air, fire, water) but its effects are conferred not only to spells of the specific type, but the special attacks of creatures with that type and weapons that deal elemental damage of that type.
Power: An elemental node has a power level of 1d4+1.
Effect: Any damaging effect or spell of the proper elemental type used within the node is increased by the Power level of the node. In addition, any creature that has resistance against element loses that resistance while within the area of the node. Immune creatures are reduced to having resistance. The saving throw DCs for all spells and abilities of the elemental type are increased by the node Power level. Creatures of the elemental type gain 5 x Power level temporary hit points while within the node and an increase to their Proficiency bonus equal to the node Power level. The area of effect of the node is often hostile to creatures not of the elemental type of the node. Every 10 minutes within the node, unless protected by some form of magic, such creatures must make a Constitution saving throw (DC 10+Power level) or gain a level of exhaustion.
Temporal Rift: This kind of arcane aura is almost always caused when great magical powers — gods, demons, or archmages of supreme power — battle in the mortal realm. Whatever other destruction their wrath causes the world, evidenced by shattered mountains and dried seas and deserts turned to glass, time itself is torn asunder. Those that wander into a temporal rift may never find their way out as they are caught in a loop of a small amount of time, doomed to repeat the same actions over and over without memory of the relative eons they have spent trapped within. From the outside, a temporal rift appears to be something of mirage, within it just the briefest glimpse of those trapped inside, so subtle and quick that the viewer can never be sure they saw anything at all. On rare occasion, an evil spellcaster or other creature finds the temporal rift and turns it into a trap for its enemies, even going so far as to build its entire fortress around the affected area.
Scale: A temporal Rift arcane aura is relatively small, just 5 feet in diameter per Power level.
Scope: Time only. Unlike other arcane auras, the Temporal Rift is very specific in effect.
Power Level: A Temporal Rift has a power level of 1-6. The higher the power level, the greater the event that spawned the Rift and the more apparent that events impact on the land for miles (1 per power level) around. At power level 6, the destruction is evident even thousands of years afterward.
Effect: Any living thing that enters the area of the Temporal Node is likely lost in a short time loop. the creature is allowed an Intelligence saving throw DC 12+Power level. On a successful save, the creature manages to pass through the rift and out again, completely losing the time it was within the Rift. To determine how long the creature was lost in the rift, roll 1d6: 1=1 round x Power level; 2= 1 minute x Power level; 3=1 hour x Power level;4=1 day x Power level;5=1 week x Power level; 6= 1 month x Power level. On a failed save, a creature is lost in the Temporal Rift forever, never to return to the normal time stream except through the use of a wish or divine intervention. Inside the Rift, the creature experiences a number of rounds equal to the Power level of the Rift, during which it may act freely. At the end of those rounds, the creature returns to the exact place and state they entered the Rift with no memory of any time passed within the Rift. Even death does not free the creature, as it just returns to its living state at the point it entered the rift once the appropriate number of rounds have passed. While within the rift, the creature is invisible and inaudible to those outside it and is considered to be on another plane of existence to determine what spells and other magical effects allow others to interact with those within the Rift.