When did “Darth” become a title?

Question

In their confrontation aboard the Death Star, Obi-Wan addresses Lord Vader thusly:

Only a master of evil, Darth.

Treating it more as a name, than a title.

DVK's answer to the question The use of 'Darth' as Vader's name, not title, in Star Wars shows that, originally, we had Darth Vader being a much less menacing figure, but with that name. So we can be fairly sure the original intent was not for that name to be a title meaning Sith Lord.

I don't recall anyone else in the original trilogy being referred to as a "Darth", so was that being a title something that came later (perhaps appropriated from the EU/Legends), or even the mention of the Sith?

So, when did Darth stop being Vader's first name and become a title?

Answer 1

In the Phantom Menace - 1999

There were no Sith other than Vader going by the title "Darth" prior to Darth Maul's introduction in The Phantom Menace released in 1999.

Before this they all used their names, i.e. Freedon Nadd, Naga Sadow, Marka Ragnos, Ludo Kressh & Exar Kun in the Tales of the Jedi series of comics - 1997.

An attempt to retcon this appears to have occurred with Naga Sadow becoming Darth Naga Sadow in 2012's The Essential Guide to Warfare

Following the release of The Phantom Menace we were treated to a slew of new Darths, including:

• Darth Bane from the Jedi Vs Sith comics - 2001, and all his successors up to Darth Sidious - It is notable, that in the Jedi Vs Sith comics, all of Bane's contemporary Sith are referred to as Lord and not Darth..
• Darth Tyrannus (aka Count Dooku) and Darth Sidious (aka Chancellor Palpatine) from Attack of the Clones - 2002.
• Darth Revan, Darth Malak, Darth Traya, Darth Nihilus and Darth Sion from the Knights of the Old Republic video game series - 2003.
• Darth Plagueis from Revenge of the Sith - 2005.
• Darth Caedus from Sacrifice in the Legacy of the Force series of books - 2007.