The Tentacle Nest Game Over is an ending which occurs when losing to the Gloomwing during the trip to Earth.
After losing, you end up getting knocked out and waking up in a small room that is made of flesh and filled with tentacles. The tentacles then grab you while a male voice (presumably a Magico Lord) explains that while you aren't exactly a Magic Girl, you'll still be of use to him. Asking for help only earns you a chuckle and an ominous reply saying you will grow to love your new room... in time. The tentacles start entering your mouth and anus while also wrapping around your penis. As you struggle, the voice tells you give in to the pleasure, although the pain received for struggling instead causes you to lose your senses.
When you wake up again, the pain is gone. However you are also extremely sensitive and covered in the tentacles secretions. When you try to think about escaping, the pain returns. The moment you stop struggling the pain disappears and nothing but the pleasure returns. Exhausted you pass out again.
Eventually, you learn if you try to escape or think about resisting you experience pain, otherwise with positive thoughts of submission only pleasure awaits. The tentacles take care of everything you need to survive while slowly changing you, leaving you always in a drugged daze. Eventually they reshape not only your mind but your body too into something much more feminine. Finally, once you have been fully turned female, your fate is explained. You have become breeding stock for this voice's minions. Since you have already embraced your fate, you spend the rest of your life loving being used by the voice's minions, lieutenants, and even himself in order to create more minions.