Challenge: the Great Convergence

*The cosmos has been shaken on its foundations, taken for a spin and blended into volatile cocktail of worlds colliding, merging, clashing and meeting! The boundaries between the ordinary and the irregular are... well... screwed up in a way that even its mother wouldn't recognise.. What does that mean for us, you ask? Well.. allow me to tell you:*

I want to propose a challenge to the talented people of r/worldbuilding. Lets do some crossovers. To participate in this challenge, briefly describe your world in 1 or 2 paragraphs maximum. After that, reply to the other comments to describe what happens when suddenly, your world and theirs would come to coexist in the same space.

The idea for this challenge came when a friend asks me why my fantasy story shouldnt have aliens (as a joke) but it actually set me to thinking. What'd happen if there were?

Hopefully we'll get some cool mixes going :D