What's the strangest terrain you've used or had a character experience in a fantasy campaign?

Well, in one game set in a variation of Lovecraft's Dreamlands, there was a huge river of boiling flesh, within which were constantly forming and melting ( or popping like a bubble) the wailing heads of infants, so that the Cacophany of screaming babies immobilized all within several metres.

Crossing this led to the shard steppes of the Stairway of Attrition, winding sideways toward the Bridge of the Gods, a huge spinal column stretching over a foul-smelling abyss that, if one could see clearly enough into the depths, is actually a mile long breeding pit of giant locusts. Their constant hum of kricking legs can lull travelers across the bridge to sleep and toppling to their doom. Occassionally the locusts would swarm, stripping the land of all life in a continent-spanning plague.

Across the bridge lie the decomposing head of The First God; a colossal beast-man whose massive bovine tongue, rotting and putrid, formed the entrance to the home of the Mindwyrms. Resembling nothing so much as a metre-long maggot with patches of sticky white cotton on their back and a maw constantly spewing forth the much sought-after Bile of Enlightenment. This pooled in the middle of the 'room', where it formed the amniotic fluid for a pink pulsating stack of human-sized eggs.

To escape, the players chose to avoid the bridge again, and braved the Forest of Not, where the trees are so tall, that several smaller forests have formed
 in levels between the branches, with ground composed of moss and vine.

Catching a ride on a giant millipede with the head of a blind pig, the players made their way down to the bottom floor of the Forest of Not, the Sea of Leaves. Strange fungi, with scabs of grey fuzz covered the floor below the wet mass of rotting leaves in various states of decay. Here lay the massive burnt trunk of The First Tree. The Copse of Yggrasil. It's said that carving a door into this trunk will create a portal to anywhere in time and space, though none now loved who knew the secret of how to achieve the destination wanted. Beneath the trunk the Lich-Squirrel waits, Ratatosk, now nothing more than the skeleton of a giant rodent with a new musculature grown from moss.

The Lavalite World from Philip Jose Farmer's "World of Tiers" books. It wasn't that much fun in play, though, and quite a lot of work to referee.

A ringworld terrain.  D&D characters we teleported to one.  Climbing down a hole and reaching a steel-plated hull freaked them out.

In a city...

...on the back of a giant beetle, that kept moving whenever it wanted or needed too.

Quote from: Silverlion;680727In a city...

...on the back of a giant beetle, that kept moving whenever it wanted or needed too.

Pretty same here, except built atop of a dragon.

I had a group go through the elemental plane of air portal in the Temple of Elemental Evil - all of them.  Rather than say "You're lost in the Elemental Plane of Air, forever, roll up new characters" we just continued it on there.  Found an "island in the sky" in which an evil cloud giant lived.  The island was an earthen torus some 10 miles in diameter around a stone tower that had a "moat" of air and was anchored by large chains.

It had been created generations ago when a high-level group of adventurers traveled to that plane and encountered the current cloud-giant's father who unlike his son was neutral-good.  The adventurers included a high-level druid who, through judicious use of various earth and weather magics was able to create an island as a place that other travelers from the prime material plane could rest upon, and the good giant king and his wife would host them.

The druid used his powers of summoning to populate the island with flora and fauna, but left the town that'd sprung up near the cloud giant's castle/tower to be on his own out in the woods.  The cloud giant's son eventually killed his father and banished his mother and enslaved the populace, with the help of the druid (who felt the whole place had become an untenable "city park" anyway, and was looking to restore the balance some).  The other adventurers were killed or fled, leaving normal types on the "island".

The druid went into seclusion and was able to hide himself and some of the refugees who'd fled - it was at that time that the adventurers showed up...

The island itself had plenty of water (but no rain; it would periodically drift through clouds that kept the land hydrated), some small bands of humanoids here and there, shallow but extensive caves (the island was many hundreds of feet "thick"), and the aforementioned village.  For the first month or so the party was there they were at -2 to all actions due to the lower gravity but adjusted over time.  They suffered a similar penalty for a while when the returned to the prime material plane.

The most fraught I've GMed in recent years was a battle on the back of the freaking CRIMSON BAT.

John Malkovich's left testicle.
ACE Agents game years ago. P.Y.T.H.O.N. had kidnapped Malkovich, who was scheduled to visit the president, and used their wacky mad science to shrink down a bunker and implant it into Malkovich's body. I can't remember if they were going to grow it to normal size or detonate the self destruct in case of failure. We recovered Malkovich and shrunk ourselves down to forcibly remove the enemy agents so the couldn't detonate the bunker. Once they were taken down the fortress was removed and Malkovich sent on his way.

Silly, stupid, but very very fun. Miss that DM. He was..... unusual.
Yes it occurred after Being John Malkovich came out.

Well, I think that one might win it.

It was memorable.

I admit I was at the next gaming table over, when the disbelieving "Jesus Christ!" from one of its players alerted us to the Cat Piss Man GM cackling about how the party was surrounded by knee-deep molten smegma, and what were they going to do now?  Huh?  Huh?  That didn't happen to one of my PCs, though, 'tis true.

(I really wish I was making this up.)

There are some things you cannot unread.

And I thought my pudding-covered battlefield was icky poo eww nasty.

Quote from: baran_i_kanu;681230John Malkovich's left testicle.
ACE Agents game years ago. P.Y.T.H.O.N. had kidnapped Malkovich, who was scheduled to visit the president, and used their wacky mad science to shrink down a bunker and implant it into Malkovich's body. I can't remember if they were going to grow it to normal size or detonate the self destruct in case of failure. We recovered Malkovich and shrunk ourselves down to forcibly remove the enemy agents so the couldn't detonate the bunker. Once they were taken down the fortress was removed and Malkovich sent on his way.

Silly, stupid, but very very fun. Miss that DM. He was..... unusual.
Yes it occurred after Being John Malkovich came out.

You win the thread.

Quote from: Opaopajr;682303And I thought my pudding-covered battlefield was icky poo eww nasty.

I can think of at least one of my players who'd perk up and ask cheerfully, "What flavor?"