I've just re-watched the entire original series of Jonny Quest.
One of my favorite episodes is called "The Invisible Monster." It left a strong impression on me as a child. It delves into the unknown and lets the audience imagine what the monster could be.
Since it's weird, purple (by the time they make it visible), and nearly unstoppable, I thought it only fitting to include it in our collective The Islands of Purple-Haunted Putrescence campaign.
So, what is it? Several scientists created the entity (or released it) during an experiment with molecular energy at a research station on the purple island known as Kravian. The research station was destroyed as soon as the monster appeared, killing all of the scientists and everyone in the vicinity.
The entity is invisible, although certain chemicals or spells might color it, giving the thing shape and form - amorphous and hideous to behold. If any part of it becomes visible, the entity will glow an unnatural purple for up to 2d4 hours before resuming its natural state of invisibility (no matter what hues were used in making it conspicuous).
The Invisible Monster
AC: 18  while invisible, otherwise AC: 14 
Attack Bonus: +10
Special: it's immune to critical hits and non-magical weapons. It takes no damage from electricity or energy weapons. It takes half damage from spells. Spells attempting to hold or restrict its movement are unlikely to work as the thing is made of pure energy.
If it scores a critical hit, the target disintegrates as if it had been struck by a sphere of annihilation.
The entity emits an unsettling sound, almost like a vibration. Everyone within 30' must succeed in a saving throw or do one of the following (randomly rolled)... 1) run away in terror, 2) throw down their weapons and worship at its "feet", 3) commit suicide, 4) attack it without mercy until either they or it ceases to be.
It regenerates 3d4 HP every round.
Treasure: since the monster destroys everything in its path, it has no treasure. However, the adventurers who take an active part in slaying the beast will automatically advance two whole levels!
Authored by Venger Satanis