Field Power Effect

"You rise with the Moon, I rise with the Sun."

—Zuko, Avatar: The Last Airbender, "The Siege of the North, Part 1"

In Real Life combatants naturally seek the high ground and other places on the battlefield with tactically advantageous Geo Effects, and to maneuver their opponents into tactically disadvantageous positions. A related tactic is to affect allied and enemy morale to make the fighting easier. In fiction the Field Power Effect is a third, supernatural way to make a given set of characters (whether heroes or villains) stronger or weaker.

A Field Power Effect is a spell or environmental condition that affects the fighting ability of one or more characters, strengthening and/or weakening them. Weather and terrain may play a factor, such as a pyrokinetic being weak during a rainstorm while an aquakinetic gains more power. Or a solar powered hero being strongest at noon and weakest when in pitch black darkness. Terrain like the Place of Power affect characters atuned to the type of energy it boosts (and it's opposite). You can probably tell that due to Elemental Rock-Paper-Scissors, any Field Power Effect that boosts one set of Elemental Powers will weaken whatever is in its opposition. This is similar to how opposing sides with the ability to use Status Buff and Status Buff Dispel can also end up canceling each other out. Other possibilities include things like time of day, year, or even the positions of planets and stars.

The reason Field Power Effects work is they somehow affect the Power Source of characters, or failing that their Soul. An area filled with Light energy naturally strengthens characters who are good aligned, while evil characters who are Weakened by the Light will suffer a De Powering. It'd take a very strong Cross-Melting Aura on the part of a villain to overpower that kind of disadvantage. The reverse is also true, an evil wizard who emanates Dark Side energy will make heroes quake and those Allergic to Evil outright faint. In both cases the character will feel soul pains.

Just to clarify, it's also possible for a Field Power Effect to just boost or drain without pairing both.

Examples of Field Power Effect include:

Anime and Manga

Happens in Digimon Adventure, when Etemon uses his Dark Network to depower the main 'mons.
Used in Inuyasha, occasionally. From dusk to dawn, on the first night of the new moon, Inuyasha turns human, returning to full power at sunrise.
During the Black Moon arc of Sailor Moon, she was subjected to a variable gravity field onboard the villain's ship. She managed to pull off Heroic Resolve and save her friends, standing (despite his increasing the gravity on her) and attacking him.
Some of the players in Saki are like that. Koromo draws strength from the night and the moon, so she's at her absolute strongest when the finals take place during a full-moon night. Yuuki is strongest during the East Wind, referring to herself as 'The Thunder of The East', while Nanpo Kazue is strongest during the South Wind. (A battle between Yuuki and Kazue has her hang on by a thread during the East Wind, and then make a huge comeback during the South Wind, winning the game by a large margin.) Initially, Nodoka also had a Field Power Bonus when playing Online, losing significant power when playing in person, but she eventually gained the ability to go into full 'Online Mode' in a live game, though it usually takes her a while...
Yu-Gi-Oh! has several dozen spells that give and remove field power bonuses and penalties. In the Duelist Kingdom arc of the manga, monsters get a 20% Field Power Bonus (actual name) to attack and defence in their preferred terrain - so bugs get it in forests, undead in wasteland, etc.
In Fullmetal Alchemist (manga and second anime), alchemy takes its power from geothermal energy. The Big Bad, Father, can block this energy, rendering alchemy useless in a given area. It does not block the similar magic of alkahestry, however, which has a different power source.

Fan Works

Doug Sangnoir's primary superpower in Drunkard's Walk -- when it doesn't produce a result that is inherently targetable, it floods an area about 110 feet in radius with whatever effect it's generating, affecting friend and foe alike. Not every result of its use is a strict example of this trope, but a number of them -- such as emotion controls, gravitic effects and so on -- certainly qualify.

Tabletop Games

Dungeons & Dragons
- 1st and 2nd Edition had a number of spells that helped groups of characters.
  - Bless: Raises morale and "to hit" rolls by 1.
  - Chant/Prayer: All attacks, damage and saving throws are at 1.
- The 2nd Edition supplement Tome of Magic had several priest spells of the War sphere that improved the performance of military units.
  - Courage, Morale and Rally all improved a unit's morale.
  - Adaptation allowed the unit to fight as if they were in their favored terrain type.
  - Selective Passage cut the movement cost (to travel through an area) in half.
  - A spell called Consumptive Field that drains hit-points from all characters in a area, potentially lethally, transferring them to the caster.
Exalted has several examples related to the Exalt type.
The spells "Hallow" and "Unhallow" from Dungeons & Dragons cause an area to favor good-aligned or evil-aligned characters respectively.
Magic: The Gathering has dozens of such spells for specific colors and creature types.

Video Games

Master of Magic has combat enchantments affecting everything on the battlefield. True Light buffs Life realm units and debuffs Death ones, Darkness does the opposite, Counter Magic tries to negate every enemy spell cast after it (both from Wizards and units, but magical ranged attacks are not spells) until wears out, Call Lightning whacks 3-5 random enemy units with Armor Piercing magical attack every turn, etc. Also, two combat "instants" with lasting area effects: Earth to Mud creates an area where all ground movement is slowed and Magic Vortex creates a wandering tornado that inflicts inescapable damage to any unit it runs over, may zap with lightning any nearby units (it's indiscriminate, but obviously can be cast into enemy ranks) and may wreck buildings if it wanders through a city's tiles.
- Also, some city enchantments have combat effects: Heavenly Light (equivalent to the True Light spell)/Cloud of Shadow (equivalent to the Darkness spell), Wall of Darkness (units not immune to illusion cannot shoot into the city), Flying Fortress (non-flying units cannot enter the city), etc. Spell Ward is supposed to block combat spells of one realm, but due to a bug (fixed in a fan patch) only blocks overland ones; it also prevents Fantastic units (whether attacking or friendly) of the blocked realm from entering the tile.
- Magical nodes have two inherent effects (shown via "Info" button on tactical map): "Dispel Non-<Realm>" (on the node's tile only) in combat works as permanent Counter Magic with exception for spells of the same magical Realm as the node (Node Mastery retort circumvents this), and "<Realm> Node Aura" (extends to adjacent tiles and becomes visible when a node is tapped by a Wizard), which gives a boost to all units of the Node's own realm (+2 to Defense, Resistance and Attack Strength of all attacks).
In the Age of Wonders games, wizards can cast certain spells that affect an entire battlefield. Examples include Holy Light, which brightens the scenery and buffs Good units and debuffs Evil units, and it's Evil counterpart, Unholy Darkness, which does the opposite.
In Battle for Wesnoth, lawful units fight better at day, and chaotic units fight better at night (and both fight worse at the opposite daytime). As well, the Mage of Light unit can illuminate an area with their presence, being a Field Power Bonus for lawful and a Field Power Drain for chaotic units at the same time.
Chrono Cross has a field effect that, when biased completely to one colour, empowers spells of the same element and weakens those of the opposite and makes Summon Magic of that element useable. Good luck doing this when enemies are using their own spells. One character has a special ability where he changes the field colors to all one color (he spills paint), but you can't choose the color. There are also normal spells that can change the field to a single color except black and white.
One of Exit Fate's Demon Commandos possesses a spell permanently buffing up his own party, and the other one has a spell Field Power Draining the enemy party.
In Fate Stay Night, both Caster and Rider use such effects to absorb energy, although we only see the effects on the field created by Rider, when it drains life power from the students of the school. Also, Ryuudou temple seems to fulfill a similar role, weakening all the servants who enter it (or so we are told).
- Reality Marbles also do this, imposing the user's Mental World on their surroundings.
Pokémon has Weather Effects that benefit specific types, moves and abilities. It can be on Routes starting from Gen. 3, or you can set it up with respective moves or abilities.
- Rain boosts Water moves, weakens Fire moves and makes Thunder and Hurricane 100% accurate.
- Sun does the opposite to Fire and Water moves, makes Solarbeam skip charging and make Moonlight, Morning Sun and Synthesis heal more HP.
- Sand hurts all Pokemon that aren't Rock, Ground, or Steel-type (or have an ability to make them immune). It also gives Rock-types a well appreciated Special Defense boost.
- Hail hurts all Pokemon that aren't Ice-type (or have an ability to make them immune) and makes Blizzard 100% accurate.
- Several abilities also have something to do with the above weather effects. They either double their speed, make them gain HP gradually, or power up their moves.
- There's also Fog which lowers Accuracy, but you can't activate it on your own.
- Shadow Sky causes gradual damage to all non-Shadow Pokemon. The reverse of Fog; it MUST be manually activated, as Shadow power is unnatural.

Western Animation

In Avatar: The Last Airbender, Firebenders and Waterbenders gain strength from the sun and the moon, respectively. Firebending is stronger during the day. Waterbending strongest at the night of a full moon.
- The sun and moon's respective eclipses render their associated bending art unusable. It's used in the Siege of the North, and fuels the plot for about 1/3 of the series.
- Fire Lord Sozin also decided to make his own Field Power Effect with a "comet" that drastically powers up Firebending. He used it to wipe out the Air Nomads, 'cept for the one they were looking for, and 100 years later, his descendant, Ozai tried to burn the whole Earth Kingdom at the comet's return.
Bionicle: the energy in Karda Nui causes beings of the Light Elemental Powers to grow larger.
The Disney Gargoyles sleep during the day, and awake at sunset. A somewhat extreme example, since they're helpless from sunrise to sunset.