We're talking massive, global mayhem?
As the sky catches fire, meteor burning flames comes crashing down through the atmosphere towards a huge game area, covering an enormous amount of land (or sea). All cities are basicly Alpha-Centauri-nuked, forests catches fires and are burnt to crisps, roads are destroyed, and thousands upon thousands, possibly millions, of people are incinerated. Entire armies are potentially destroyed; possibly even heroes if they suffer a direct hit. Any city that are struck directly by a stray ember could have entire squares temporarily destroyed, or/and city improvements permanently destroyed.
The sun stings an immense area (quite possibly entire empires), drying up wells, slowly baking people in a murderous, never-ending sun. Farmland & villages suffer from severe famine, forest fires could start after a prolonged duration, and grass withers into savannah and ultimately, desert. Can be counteracted at great cost. A double-edged sword if aimed poorly. Can be protected against in areas (cities, etc) with magic.
A wide area becomes prosperous. Birth rates of all creatures (living resources such as horses, bears, unicorns or chocobos increase) increase rapidly and at first exponentially (but later slowly tops off, but never ceases). Hills become teeming with life, rivers filled with fish and crops can be harvested more than once a year as yields are quadrupled.
A earth-based version of Starfire, with much higher material loss rather than human lives. Causes a massive earthquake. Armies in the field and such are mostly unscathed, but cities and structures crumble and mines collapse. In the aftermath; from the forests, the rivers, the canyons, the hills and all around, nature rises up in fury of this transgression, attacking any living thing (sometimes even eachother). Earthquakes may unveil (or collapse) mines.
Creates a canyon. An actual canyon, that may or may not be filled with water, tearing land from land. Anything in the area of effect is lost forever. Has range restrictions (although the range is massive) and can be defended against with area-target magic shielding (including city improvements to prevent the capital or major cities from being completely destroyed). Also causes at least minor earthquakes in an area around the area of effect (where the actual canyon is created). Earthquakes may unveil (or collapse) mines.
Not "global mayhem" but I want to mention it anyway. Raises structure X from the earth itself, built in materials and style appropriate to the surrounding land(s). In most cases, this would be stone & earth, but in a forest it may be entirely made out of wood, or coastland, coral. In hills or mountains, a chance for crystals. Not usable to build cities, since cities require an entire infrastructure, with people to go with it. This just raises the structure. Fortresses, Fortified mines, Node-fortification, Temples, are all possible.
Fabrics of Reality
Tears a hole in the fabric of reality, slowly, turn-by-turn, devouring everything around it. Spreads to a certain size (depending on mana spent). Permanent until dispelled. Destroys everything, permanently, leaving nothing, unless magically defended against. When dispelled, leaves nothing but fire-scorched, magic-drained, infertile land.
Mountain of a Thousand Winds
Tears a piece of land from the earth (filling the hole with water), floating it high into the air in the visage of a turned-around mountain. A city can be built on the land; Requires special equipment to have supplied by caravans, but flying creatures such as pegasi or gryphons are more likely to settle (and become resources) there, and also consider it better habitats (higher resource rate). Can be moved with concentrated use of magic, or have special improvements built in it to allow it to move, slowly, slowly, slowly. Can never leave the magical sphere of influence of the channeler (with the Channeler actually having the potential of being IN the city itself, circumventing this limitation). Can also be dispelled by a strong rivaling channeler (or the own channeler, if he wants) sending the land crashing down.
A violent, virulent plague spreads across the lands, feeding off magic and spirit alike, rapidly killing populations from the point of origin and outwards at an incredible rate. Can't be dispelled, but can be counteracted by having the proper improvements or throw the proper (healing/regenerating) spells. Devestates affected armies and city populations, and renders magic equipment (on armies and heroes alike) useless for the duration (if they are affected). Spreads by primarily along roads, from the point of origin, moving much slower in non-cities/non-roads & uncivilized areas. Watch how quickly the enemy can move their troops out of the cities and away from the roads.
Turns all mount-resources (Horses, Unicorns, Chocobo, whatever) into Bears. All of them, within the Channeler's empire. Also doubles their production rate. Rawr!
That's all I feel like penning at the moment, although I could probably come up with thousands of these.