Here are some encounters that you could run in a truly alien setting, like another planet or a far realm. They’re pretty low-level, suitable for characters around level 5 to 10, or for, say, level 3 characters who are in way over their heads.
These encounters are mostly inspired by weird stuff I saw in a documentary about coral reefs.
Roll d12 on this chart while traveling, or d6 while stationary. This encounter table is based on this encounter table template which incorporates rules for weather, resting, and getting lost:
1: Make up an encounter that’s related to your campaign or to one of your characters. If nothing springs to mind: you meet a plane-traveling evil level-15 wizard from the world of Mystara. He/she is searching for a Sea Anemone of the Magi said to be lost in this dimension, and suspects the PCs of trying to get it first. He/she will not attack unless that suspicion becomes a certainty. He/she will trade help or information, but offers nothing for free. His/her skin is covered with weird flowers and he/she detects magic at will (see encounter 4).
2: 2d4 levitating, hand-sized, ovoid stars which cast bright and cheery light in all colors. Their touch is telepathic poison: saving throw or take d20 psychic damage from their nihilistic world view. Evil characters take no damage. AC 15, 2d8 HP.
3: 1d4 walking ten-foot-tall antlers. They charge and impale victims for 2d20 damage. Impaled creatures take full damage each round until they use an action to disentangle themselves. The antler can carry around up to 3 impaled creatures at a time. AC 18, immune to piercing attacks, 10d8 HP.
4: A field of human-sized flowers which snatch victims in their finger-petals, digesting them for 1d20 damage in subsequent turns. AC 12, 1d8 HP. The flowers’ only sense is Detect Magic, so they will only attack creatures under magical effects or with magical gear. Anyone who takes damage from a flower will sprout small, harmless flowers in 1d4 days. Until the flowers are pruned, the character can Detect Magic at will.
5: 1d4 quivering, jelly-like, transparent pillars, periodically shot with neon lightning. They travel like Slinkies. 2 in 6 chance that there is a sluggish creature inside (lumpy cone, fire-spewing sac, silver torpedo, teleporting lightning bolt). If any players push their way into the pillar, they are rewarded by being healed 1 HP for every 10 minutes they remain inside. AC 10, 10d8 HP, regenerate 1/turn.
6: Rot grub rain. Anyone whose flesh is exposed to the rain is infected.
7: Rotting, gooey meat surrounds a 10-foot-wide hole in the ground. Inside a three-room underground lair is a giant armor-plated shark (AC 18, HP 20d8) with a giant, exposed brain (AC 13). Called attacks on the brain do double damage but the attacker must make a saving throw or take 1d20 psychic damage. The shark can make a bite attack (3d20 damage) and a gaze attack at a different target (gaze: saving throw or the target and its equipment becomes soft and gooey: AC reduced by 5 until the target next eats). The lair contains 1d6x1000 GP worth of cut gems, a gem vein with a potential 5d6x1000 GP more in gems, and inch-tall miner octopi (each day, they mine and cut 250 GP worth of gems, of which they eat half).
8: A silent wave of purple light rolls over the sky. In every direction, the sky and ground are twisted and swirled, so the shortest path between any two points is a curve or spiral. Travel requires a daily DC 15 Intelligence check or the party is lost. This effect lasts for 1d4 days or until someone gets a natural 20 on one of the intelligence checks.
9: 1d4+1 pools filled with a heavy oil (if burned, burns blue and does cold damage) and small, oily, scaly creatures. If you dive into the pool and swim through a thirty-foot-long tunnel, you will emerge from a different pool. (Roll 1d6: 1-3: the exit pool is more than 100 miles away on the same plane. 4: the exit is somehow the same as the entrance pool. 5: The exit is on a different plane. 6: The exit is on the swimmer’s home plane.)
10: A waving fern which exudes a visible bubble, 30-foot radius. The bubble is buzzing with small, flying, colorful creatures of all descriptions. Any creature inside the bubble is under the protection of a Sanctuary spell until it does violence.
11: A mountain of curvilinear catacombs, terraced apartments, and caves of chaos. At the top of the mountain, inch-tall blind octopi labor to create new levels.
12: Inch-thick tubes of multi-colored, hyperdense stone (20x the weight of normal stone) lead in an intermittent trail to a human-length, flat, spined worm. Rear attack: poison spines, 1d20 damage, saving throw or paralysis for 1 minute. Front attack: The worm’s all-consuming maw telekinetically sucks in everything in front of it in a 20-foot cone (save or be swallowed) and excretes everything (including adventurers) as inch-thick stone tubes. Stone to Flesh restores things to their original shape, but dead. AC 20, HP 15d10.
Wow – rot grub rain! I love these imaginative ideas. I think some of the best encounters are dreamlike like this.
I also like no’s 2, 3 and 5. In my campaign there’s a cursed land, a place of ancient battles and where the most terrifying magic was unleashed, and some of these effects could add a lot of colour.