So here's my thought process as I came up with this particular monster.
Last week, I'm thinking, it would be cool to have a section of the megadungeon have gas spores. I never use gas spores. I'll put in dozens of them. Dozens will serenely float towards the players, waiting to pop and spread spore-y death.
Then I thought about it a bit more, and it occurred to me that my players would think I was an imbecile to put dozens of gas spores in. A conga line of beholders heading towards them. They'd probably think they really were beholders, given all the pep talks about dungeon deadliness I've had with them. And really, it is kind of a stupid idea. But now I'm focused on floating orbs, lots of floating orbs. Like balloons, or bubbles.
Yeah, I'll have evil bubbles that attack the characters. They won't look like beholders, so now it's cool. How threatening is a bubble though? They'll just bounce off the guy's head? How would that even hurt? I guess they'll be electrical. No, they'll absorb life force. Like negative energy or something. But I hate negative energy. Whatever, they damage people by touching them. But how hard can it be to avoid a bubble? It's a big round thing. Nobody would get hit by that.
I know, they'll have tentacles. No, then they won't be bubbles, they'll be tentacle things. I want a bubble. OK, if it's a bubble, it can probably change shapes. So it can whip out something to touch the players. Right, a whip of bubbly death.
And after this derailed train of thought, I present to you an utterly silly monster, the Malignant Sphere. I'm probably going to use them anyways. Speak up and let me know if it's too stupid though.
No. Enc: 4d6
Movement: 60' (20')
Armor Class: 6
Hit Dice: 1d4 hit points
Hoard Class: None
The malignant sphere appears to be a translucent, iridescent bubble, two feet in diameter. It moves by floating through the air, and attacks by extruding a thin tendril, that it stabs at its opponent. Contact with the sphere or its tendril causes 1d4 points of damage, as it absorbs its victim’s life force. The sphere is incapable of harming undead or constructs, only living creatures.
If the tendril is damaged, it will merely absorb the damaged tendril and slash out with a newly-extruded one.
The sphere is a hollow bubble, and when slain, will audibly pop, and drop to the ground as a thin puddle of iridescent slime.
A preferred tactic of the malignant sphere is to hover close to the ceiling near doorways, waiting to drop down on unsuspecting adventurers.