Ndtyhij

Another planet seems unlikely, given the time frame. Even Pluto's orbit is 'only' about 500 years, a far cry from 20.000. Also, it would not explain why the hostile period is only a thousand years.


Ice ages might be a better solution, though ice ages on Earth have lasted from ca. 50,000 years up to a hundred million years. Nor are they very regular.



Poul Anderson's 1974 novel Fire Time has a planet that experiences a period of extreme heat every thousand years due to the planet's irregular orbit around three suns. This might be tweaked to suit your needs.



Perhaps the best fit might be if is your planet and its sun is in a very eccentric orbit around a far larger sun, or possibly a black hole with a accretion disk emitting hard radiation. A characteristic of eccentric orbits is that most of the time, the orbiting object is far away from the larger object it orbits and spends a relatively short time whizzing close past that object before returning to a farther distance. An example is the comet Ikeya-Seki, which has an eccentricity very close to 1 (which is the limit for orbiting objects). Its aphelion (furthest distance from the sun) is roughly 200 times that of the Earth, but when it last came closest, it came within 450,000 km of the sun's surface - not much more than the distance between the Earth and the Moon. Its orbital period is roughly a million years. This is far more extreme than what you need, but shows that it is possible.



Finally, since your planet is a fantasy planet, there might be a fantasy explanation. It may be that the dragons breed every 20,000 years and get a bit rowdy during that time. Or there may be a bit more exotic explanation like in the Earthdawn role-playing game, which has a recurring Time of Horrors.

I am slightly skeptical that you could have a stable system involving a doomsday planet that comes close enough to cause tidal heating without seriously disrupting the orbit of both planets after the first pass. Its the sort of thing that might cause any moons of your inhabited world to leave for somewhere more gravitationally friendly.

Now, I am no orbital mechanic, but I think that a more plausible means of periodic catastrophes are meteorite showers. One source might be gas giant planets in the outer solar system having an orbital resonance that periodically disrupts the local equivalent of the asteroid or Kuiper belt, flinging a bunch of comets and asteroids into the inner solar system some of which end up falling onto your unfortunate world.

Big asteroid impacts are obviously Quite Bad in themselves, but they are also linked to major volcanic events too, if that's important to you.

Biological Infestation

Have you read the Pern books by Anne McCaffrey? The series is science fiction, though much of it is indistinguishable from fantasy, as it's a colonial world that has regressed technologically and depends on fire breathing 'dragons'.

One of the major environmental issues in the Pern books is an organism called Thread, which floats onto the planet every time a rogue planet, with a highly elliptical orbit, passes near Pern. The Thread organisms consume pretty much anything organic they touch.

Obviously you wouldn't want to directly copy this, but it could be a good jump start on thinking about other ideas.

While I'm inclined to favor the orbit-involves-something-nasty answers (note that the nasty object could also be a fast-spinning neutron star, the jets can be powerful) you've got a far bigger problem here:

After you have wiped out the land breeders the first time there will be no more land breeders. Your period between encounters is nowhere near enough for new land breeders to evolve.

If you want to have a repeatable pattern occuring every few thousand years I would suggest a biological reason rather than geological/astronomical. This way you can explain that the cycle is a part of a wider pattern in the ecosystem which lets you imagine more complex situations than just "boom! everything is destroyed! start to grow legs again, you stupid fish!" ;)

We are used to think that ecosystems are stable, but that's only because we have a very limited timeframe. Even though we know that there was a very different climate just twenty thousand years ago (and that now climate changes are even faster), our intuition tells us that summers and winters should be similar each year and nothing is really changing unless there is some kind of a catastrophic event. But that's only because our lifespan is 100 years tops. The ecosystem works on grander scale and doesn't need any meteorites or global volcano eruptions to go through different phases. It is also possible that there are intervals of short rapid changes followed by a long time of considerably more stable situation.

I would start with the assumption that the most complex life on the planet is amphibian or that all species which resemble mammals and birds are capable of spending their whole lives on water if ground is too hostile. When the conditions on ground improve, those species are able to quickly colonize the new ecological niche. But since a few thousand years is not enough for evolution, all those species will be dependent on water: so no complex animals in steppes, deserts, mountain plateaus, etc. That part of the world is left for a very different ecosystem, one composed of plants and animals which are smaller, simpler, but because of that they can multiply and spread very quickly when they reach certain "critical mass".
Think of it as a biological version of a nuclear bomb. We start with a very desolate place, a desert far from water. There are some species of plants and animals there, but they're rare and live in a fragile balance: plants grow, herbivores eat them, carnivores eat herbivores, less herbivores mean more place for plants to grow, but also carnivores starve, and this small cycle of small organisms (bugs? insects? miniature tentacle monsters from hell) repeats itself. But this is not really a balance. Every cycle the average population grows a little. And with bigger population, every cycle is a little bit more violent. At some point we reach the critical mass and everything goes boom - in the biological sense. The plants of this strange violent ecosystem spread rapidly around the globe. Herbivores follow, but eating not only the plants they know, but also one which survived in the waters and only recently started to colonize the shores. Carnivores follow, but hunting not only herbivores they know, but all the complex animal life which survived the last cataclysm living in the sea. The only way for the complex ecosystem to survive is to hide in the sea again: the miniature tentacle monsters (*) cannot be stopped. But when they finally eat everything in their way, they die of starvation. And since the cycle was so violent this time, only a very small number of the species form their ecosystem survive - somewhere far away from the shores, on the desert, starting a new small cycle within a bigger cycle which takes thousands of years.

*) I think it's too easy to think of them as a swarm of insects. I'd invite you to imagine something different.