Darth Momin is an ancient Sith Lord whose consciousness persists inside his mask, introduced by Charles Soule, Giuseppe Camuncoli and Daniele Orlandini in Star Wars: Darth Vader #21 (Marvel Comics, 2018). Palpatine gives the mask to Vader as a tool for building the fortress on Mustafar.
The Sith as artist
Momin's premise is the most genuinely novel thing anyone has done with a Sith in the modern comics: he is not a general or a politician but an artist, and the dark side is his material. His masterworks are constructions and events, and the reason he was executed by his own master was aesthetic ambition rather than treachery.
Design of the fortress
The arc uses him to explain the tower on Mustafar seen in Rogue One — a structure Momin designs as a machine for tearing open the veil between life and death, which is what Vader actually wants it for. It is a rare case of a comic retroactively explaining a film's set dressing and improving it.
Continuity note
Neither his species nor his homeworld is established in the material, and both fields are left blank here. The "Darth" title places him somewhere in the long Sith history the canon comics have begun sketching, but canon has deliberately not fixed a lineage or a date.
Canon status
Canon. Momin is a 2018 creation for the Marvel line and has no Legends counterpart.