Rethen Portal is entrance to the another dimension that is not in our classic Box. Yeah, in Box everything does exist, unimagneable and imaginable, possible and impossible but Rethen Portal redirects us to the another "Box". Box is under quotation marks because this isn't something like our where do we live, this is irregular shape that is big as about 17 trillion Omniverses.
Appears when sidronium(chemical element from galaxy Phoexobia) burns, then it creates cracks and plasma(extremly hot flame) in empty space.
When you're standing between two dimensions more than 2 minutes you will fall to The Outside. This is one of extremly rare ways to get to The Outside.