Lord Darth Vader fought the former Chief Librarian Jocasta Nu in the Temple, only learning of the vault's existence when Nu fled into it. After killing the Jedi, Vader informed his master, Darth Sidious, of the vault and the Sith Lord took the helmet into his collection of Sith artifacts. Being that Momin's helmet has not been recorded in any holocrons Jedi or Sith (as the Sith deemed him a heretic), Momin revealed and shared with Sidious about his past. With Momin given to Vader to help construct his new castle.

After the moment he nearly baited into making you wear the helmet his consciousness resided in thus possessing your physical completely under his control, you managed to capture a Mustafarian as vessel for Momin instead, to use as the host to walk and interact with everything around him.

The two returned to the cave, it was an ancient Sith cave located deep beneath the surface of Mustafar. It served as a locus for the dark side of the Force. Built by the ancient Sith as a shrine to seek the secrets of eternal life, the cave resided in the course of one of the planet's lava rivers on the site of a former Sith temple in the Gahenn Plains, and was strong with dark side energies where Momin took a moment to explore, reading it's ancient inscriptions of the Sith cave.

"Ah, so I am still on Mustafar. This planet was not always like this. Did you know that?"

His voice sounded like a whispering echo as he speaks to you. He knew Vader has a quest to seek his late wife Padmé, it wasn't just for a castle... He needed this castle for this woman, and he will have to get it from Momin himself.

As for you, you nearly fell into his trap, you weren't going to play and fall for his little games again this time. Standing a few feet behind him. Betrayal would mean Momin will suffer dearly for it.