Fantasy Aliens

I use lots of alien races on Fletnern (my FRPG world). None of them showed up through the use of space ships; they always have some magical or rarely mentalism way of traveling here. I find it a great way to add some really odd/crazy creatures into the mix without having to justify how they survive in the environment. But, to my way of thinking, if I want to have them show up, I have to justify why they show up. Why did they leave their home world and come to Fletnern? Well, the less intelligent ones normally wander through an open “portal”. You see, I designed this “spiral staircase” that all the worlds are “on”. The worlds are not actually on the spirals, but they appear to be due to some 20+ dimensional mathematics. On different but related planes of existence, a double helix of worlds is spinning in opposite directions. Imagine a piece of DNA where one side of the double helix spun to the right while the other spun to the left, all the time revolving around a central sphere. Here’s the best part - you don’t really have to picture it! The result is that every so often, certain worlds come so close to Fletnern that a person can cross from one to the other. Sometimes this is done accidently. Sometimes powerful magics are used to craft bridges that can stay in place for months. That’s how the aliens typically get here. So why do they come if it isn’t an accident? I think I’ve overdone the whole running away from something else scenario. Lots of the aliens are or were refugees, either from a natural disaster or from conquerors on their home world. To say that a conqueror would come to a world simply to conquer it seems like an immature reason, though some conquerors are immature. Some come for the slaves. Dragging alien creatures back to your world to work in the most dangerous mines (or whatever) seems a far better reason than simply conquering. Others need to gather resources. Admittedly, those resources might be slaves too (for food this time). You’ve seen all the sci-fi movies - they come for gold, for water, to steal our atmosphere, whatever. I do think gold and diamonds are reasonable excuses to go world hopping because of their rarity, plus it puts them directly at odds with the locals who are not willing to give up their gold and diamonds. Why use them? Because they are so “alien”. They can have alien powers. They can use materials that are either unheard of in your main world or so rare in your main world as to be never considered as a building material. I had one alien race from a volcanic desert world using obsidian for all their containers, as well as for weapons, eating utensils, mirrors, needles, etc etc. Best - some of these materials might give them magical items that either cannot be reproduced or will “wear-out”. Maybe those spears shoot lightning bolts, but only three or four times, so even if they wind up in the hands of the PCs, they won’t change the game balance. I like cool stuff like that!