I'm thinking that Concert Casting rules makes it possible to introduce the epic spells that sometimes appear in fantasy fiction - this type of magic typically involves a ritual or ceremony with multiple participants. A good example might be the Turning from Andre Norton's Witch World series (where the Witches of Estcarp attempt to raise a new mountain range, but fumble their roll...badly).
The ritual spells that use concert casting should be extremely rare and gaining access to them should be very difficult. For example, the characters might need to recover a lost grimoire from demon-haunted ruins deep within the southern jungles in order to acquire a single spell of this nature. Gaining access to one of these spells should be the focus of an entire adventure. In many campaign settings, they represent the fragments of ancient lore from a time when magic was more powerful.
Here are a few (very) rough ideas for the kind of new spells that could use these rules:
- Rain of Fire - This spell is similar to the Rain of Blood spell, but causes droplets of fire to rain down upon the area of effect.
- Earthquake - Causes a powerful earthquake that topples buildings within a certain radius. This spell should be potent enough to flatten an entire city when cast by a large concert. This may be the spell cast by Xaltotun at the opening of Robert E. Howard's Hour of the Dragon (aka Conan the Conqueror).
- Tsunami - Summons a massive tidal wave that devastates a coastal region, destroying port cities and sweeping far inland.
- Legion of the Dead - Raises all of the corpses in a graveyard or on a battlefield as animated skeletons and forces them to obey the leader of the concert.
- Howl of the Damned - Summons all of the ghouls in a geographical region, providing the leader of the concert with control over them for a single night.
- Hellgate - Opens a portal to the netherworld that spews forth a seemingly endless horde of minor demons. The demons devastate the area around the open hellgate, but cannot travel more than a kilometer or two from the place where they emerged or they will automatically discorporate - returning to the depths of hell.
- Wraithstorm - Causes hundreds of vengeful spirits to erupt from a rift to the sprit world, temporarily manifesting on the material plane.
- Plague Wind - Inflicts a terrible plague upon the inhabitants of a region. The pestilence spreads rapidly, striking down rich and poor alike. Extremely contagious, it is incurable by normal means. However, only those within the area of effect at the time when the spell was cast
- Divine Wrath - Invokes the wrath of a deity to smite a location in a display of raw divine power. The biblical destruction of Sodom and Gomorrah is a good example of how this would work in practice - maybe the spell can only be used against cities or temples that have fallen into apostasy?
- Black Eclipse - Summons a cloud of darkness that blots out the sun, allowing unclean things such as vampires to walk abroad during the day. While the unnatural eclipse remains in the sky, the power of necromantic spells is enhanced and the power of healing spells is diminished.
- Undead Colossus - Melds together dozens of corpses to create a huge undead colossus capable of destroying castles and other fortified structures - I am thinking of the monstrosity created by Nathaire and his apprentices in the Clark Ashton Smith short story The Colossus of Ylourgne