There are apparently more than one dark forge world in the galaxy. Xana II is one of the most famous and is specialized in the conception of advanced aircraft for chaos warbands. But there are numerous others (Samech, Chaeroneia, and others with names even less pronounceable). However, chaotic forge worlds are extremely rare (much, much rarer than their imperial counterparts anyway), which means that they possess a great influence over other chaotic followers and their services are sought by all chaotic warbands across the galaxy.
Indeed, it is only thanks to the Dark Mechanicus and its forge worlds that the traitor forces are supplied in advanced wargear. Indeed, while most heretic forces rely on looting and pillaging to get their hands on basic weapons and ammo, there are some pieces of tech that are simply too advanced or rare to be obtained via looting (
TDA suits, titans, spaceships, daemon engines, etc).
In effect the Dark mechanicum is known for producing pieces of technology even as advanced as terminator armors and titans. However, Those pieces of tech are so difficult to produce that chaotic forces usually prefer to salvage this kind of hardware on the battlefield and then send the bits to the DarkMech.
But the DarkMech does not simply limit itself to salvaging or copying imperial wargear, it also develops new pieces of technology (often thanks to their studies of xenos and chaotic stuff, which is forbidden to the AdMech), such as daemon engines (Brass Scorpion, Soul Grinder, Defiler, etc), aircraft (Hell Blade, Hell Talon, Locust, super-heavy bombers...) or even spaceships (Infidel and Idolator raiders, Iconoclast destroyer) such as the mighty Planet Killer (they already built it two times

).