On every games I made so far, there's a yellow empire called ENFER AI which is located in Incandescence.

When you enter that system you are automatically sieging it.

When you invade it the ground battle tactics they use is different than the minor factions one, it says "MIGHT BE NEENED" and has no malus neither bonus (the tactic they use is "Shoot on Sight").

The local populations are the Mavros (basically evil sophons) and inhabits one planet (boreal/cold/tiny/max 5 pop) on a one (the maximum planet of the system is 1). The population also never grow during the game.

Any toughts about this strange not repertoried empire?

It's from the Sophon Quest line, either the AI or the other player is advancing towards the end of that particular quest