With all my writing about super powers for World War Two settings I somehow left invisibility out of the posts. Last night I was watching a great flick on the late show: Invisible Agent. The grandson of the Invisible Man uses his grandfather's formula to become a super agent for the Allies.
Invisibility is a great power for a low power supers setting. You're not bulletproof or superstrong. In fact it has some limitations. Let's look at invisible men.
In the original story Griffith used a serum and an electrical treatment to become invisible. Unfortunately he soon became unhinged and went on a rampage through a small village before ... well read the story. In the movie his grandson insists the serum only be used on him (no mention of the electrical effects -maybe Griffith concluded they were not needed after all or had an improved serum in reserve.) In the movie he suffers from mood swings and slight paranoia as well as bouts of 'feeling high' and sudden exhaustion.
So you wouldn't want an army of invisible men. Better to have only a few agents (or one) since there are some side effects. Also an invisible agent is even more effective if there's no reason to suspect one. The effects of the serum last several days in the movie but either wear off or are neutralized by the end of the movie so perhaps the electrical effects both unhinged Griffith and made the serum's effects permanent.
Low Powered Invisibility- the character rolls Dexterity to avoid ranged attacks with advantage (take the lowest die). On turns they do not attack Ranged attacks can't be made at greater than nearby range. Melee attacks are also avoided using a Dexterity roll. Four is subtracted from this roll. Every time an invisible character makes a melee attack the modifier decreases by one. So after the character punches a Gestapo agent, next turn his modifier is only three Every turn they make no attacks the modifier goes up by one to a maximum of four. Usage die d6
Paranormal Invisibility- the effects are as above but the usage die is d8.
Superhuman Invisibility- the effects are as above but the usage die is d10.
A referee can ask for a Wisdom or Dexterity roll to avoid certain hazards that make the character show up. For example a Dexterity check would let a nimble invisible man to get out of the rain before he became a wet silhouette. Smoke, paint and snow could all make an invisible man show up. The usage die should be rolled every day. When it is used up the character returns to normal.
If the usage die comes up 1 or 2 the serum has side effects within 1d12 hours as follows:
1-2 Lethargy: The character must make a Constitution roll to stay conscious. They are -2 to Dexterity and Wisdom rolls until they get eight hours sleep.
3-4 Paranoia: The character becomes very defensive or downright hostile and must make a Wisdom roll whenever diplomacy is called for or fly into a rage The referee may require a Wisdom roll to flee a fight.
5-6 Insanity: The character succumbs to delusions of his own power and importance and the inability of anyone to stop him. As a bonus he gains 1d6 hit points and does +1 unarmed or improvised damage. The character needs to be restrained given therapy and a neutralizing compound. He no longer suffers from Lethargy or Paranoia results. The insanity lasts until the serum wears off. The referee can take over the character and run them as a truly dangerous NPC for several hours at a time.
The electrical treatment the first Invisible Man used makes the serum permanent until a neutralizing compound is injected. The power's usage die is still rolled but when it is used up the character
automatically becomes insane and a referee's character.
It's up to the referee how infra red sensors are effected by invisibility. In WW2 such devices were not common or easily portable.
Invisibility has drawbacks. You have to keep scrupulously clean. Food consumed does not turn invisible immediately (make a Constitution roll every five minutes to absorb the food.) Cagey agents will stick to clear broth. Finally if you're invisible you're naked as a jaybird, exposed to the elements and can't carry any neat gear. Invisible people trying on clothes can make 1 HD opponents flee in terror by making a Charisma save though the effect doesn't last long.