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I'm developing a story about a few hundred modern day US Marines who are transported to a fantasy world and must do battle with an army of Orcs and dragons. I know they will be extremely effective at this (see: literally thousands of other questions). However, to keep things from being too easy on them I'm gonna use magic to even the odds a bit.

In the world they are taken to, wizards play an important role in combat, but are NOT so over powered that it becomes pointless to raise huge armies of knights and peasants. Therefore, I need to come up with spells and types of magic that would be useful against my few-hundred marines, but would NOT utterly destroy a standard fantasy army of knights, spearmen, orcs, dragons, and eagle-riding elves.

A few I've come up with:

• 
  Stun/Disable all troops in an area: stunning or disabling a few dozen spearmen would be powerful in a fantasy battle, but not
  necessarily decisive. However, with the marines relying on a small
  number of elite troops with massive firepower, disabling most of them
  for a short time could be quite powerful.


• 
  Interfere with visibility: Clouding the battlefield such that visibility is vastly reduced would be useful in a medieval-style battle, but would not prevent a melee army from being effective. However, by preventing the Marines from aiming it could completely disable their primary advantage.


• 
  Controlling/Possesing: Being able to possess a single knight or dragon is good, but not all that OP. Being able to possess the gunner of an M1 Abrams tank could prove far more disastrous.

Can people think of any other spells that might prove effective here? Thanks!

An easy one would be an area of effect spell that negates combustion within the zone of effect. This would cause firearms to stop working and almost all vehicles would be unable to run. They would be virtually defenceless.

Another good one is a creating a localized EMP (electromagnetic pulse) to knock out enemy electronics and equipment.

With those two spells alone you could decimate a modern armies land,naval and air superiority,cut off the majority of communications instantly,likely cripple morale and leave them unable to defend themselves adequately.

If you wanted you could also use animate object spells on vehicles. Then suddenly your vehicles are all essentially enemy drone units that need to be destroyed by your own forces. In a medieval era such a spell is dubiously useful in warfare,but the era of mechanized warfare would change that.

Hope you found these suggestions helpful.

Your question has multiple angles that have to been considered. We not only need a magical way of dealing with our modern tech, but we need a reason for this spell to exist before the modern tech existed, and we need this method to not be overly effective against typical medieval combat.

One idea would for there to be magic for dealing with specific large threats: I'm talking about dragons.

Dragons use fire as a primary weapon. Marines use explosives, which utilize fire, as a primary weapon. A spell to nullify fire in an area would be very useful against a dragon, but nigh-useless against even a peasant with a pitchfork. This spell would also have the side effect of rendering gunfire, rockets, and other similar modern weapons useless.

Dragons fly. Marines might have helicopters or other aircraft. A spell to ground flying targets would be very useful against dragons and helicopters alike. It would be useless against a ground army.

Dragons move very fast. Bullets, missiles, mortars, ATVs, Humvees, Tanks, etc also move very fast. A spell to impose a speed limit on an area would be highly effective against a dragon. It would be devastating to marines. It would have little to no effect on pike-men marching across a field. It might help, to a limited extent, against archers (although the higher mass of arrows would maintain effectiveness even at lower speeds... and arrows go much slower than bullets and missiles anyway).

For the sake of your story, I'd let these kinds of spells be somewhat esoteric. They're not useful on a day to day basis, or for most common threats of the world. A given wizard might know ONE of these spells, and that's all they'd need to aid their local knights in taking down the rare draconic threat. This would mean that your marines will have to adapt their tactics and methods based on which anti-dragon spell your local wizard is familiar with at the time.

Animate Dead

Bullets are not as effective on the dead as swords or axes. If someone falls in the fantasy army, they get back up and keep going. If someone falls in the modern army, they attack the soldiers around then causing fear and dread.

Drain batteries

Render all lasers, radios, vehicles, and computers useless

Shrink Lead 10%

Reduce the muzzle velocity, range, and accuracy of bullets.

Rangar Lalazar was a third rate sorcerer from a rural backwater in the early first century. He is mainly mentioned in your first year magic textbooks for his invention of the "water to oil" transmutation spell. It is a good first level transmutation spell that is easy to learn, is emminently useful (for replenishing lamps during late night study sessions) and has very few ways to go wrong. The most notable of which merely causes nearby surfaces to become slippery. However few now remember that he also contributed this spell's opposite to our magical lexicon. So when our scouts reported that the strange metal beasts of our enemies appear to consume oil I directed our wizards to ambush them in the mountains with the "oil to water" transmutation and as they say, the rest is history.