"Gory, gory, what a hell of a way to die,Gory, gory, what a hell of a way to die,He ain't gonna jump no more!"
- – Blood on the Risers
The Phantine Skyborne are some of the rarest sights in the Imperium, and on the table top since they are the third Air born regiment Game workshops made. With less information than the Sable Swords or the Segmentum Pacificus, they are perhaps THE most unknown regiment in the imperium. Heck, even regiments like the Ritterghast or the Janiverden Tunnelrats have some mention in recent codexes. Part of this is that the formation of the unit it self is relatively new. Most of the men tithed to the Imperial guard from Phantine in the past did not serve as infantry, but as pilots in the Phantine Air Corps. Phantine's can almost be thought of as a reverse Barbarus, toxic gases and corrosive fogs are in the lower atmosphere, so the population lives on the tops of mountains. With no real ground to move and train on it was natural to raise units of pilots to play to the specialty of a population where that was the only way to move from one city to another. But the PDF on the other hand learned how to fight as drop troops and airborne forces as they had to be the boots on the ground in that environment. In times of need these PDF forces can be 'promoted' into Imperial guardsmen which was done during the Sabbat Worlds Crusade where they become Phantine Skyborne.
Aside from that there is not a lot of information about them. But based on what we do not know they are among the higher quality and better led regiments in the Imperium. We can guess this since we have not seen countless regiments fall in some doomed infantry advance, in some desolate corrupted world for no reason, which means that among the guard, they are likely well trained and well managed regiments in the Imperium. That is no small feat but then again no large one either.
There is a bit of Skub regarding if they were a regiment inspired by the "SAS" british commandos since they're an Airdrop unit, trying to be more distinct from the other two airborn regiments, Elysian and the Warhawks. The Elysian are more traditional paratroopers, while the Warhawks are basically storm troopers which leaves the Skyborn in a bit of an odd position. Since about 1/4 of the real world S.A.S are mountain troops, and they're also known for paradropping the assertion about whether or not they were inspired by the SAS is a logical, if somewhat unsupported given what we know about them, which is again not much. The fact that they are elite paratroops for the Phantine Airborne, which are Battle of Britain RAF IN SPESS, also gives weight to the SAS idea.
The real answer to the question about if they 'are' or 'are not' SAS depends on how one models and converts one's own Skyborne, the Skyborne would be 'your dudes' and if you want to make them based on the SAS, go head.
Why Play Phantine Skyborne?
While there are at least two more widely known air born Imperial Guard Regiments in the form of Elysian Drop Troops and the Harakoni Warhawks, both those guard units have a more set look, feel and appearance. If you want an airborne regiment that you more freely model and convert to fit your backstory, and your reason idea for your army, then the Skyborne are a good choice. The Skyborne are a good fit if you also want to use planes from the Phantine Air Corps and so you can have a fluffy airborn army with close air support.
Sabbat Worlds Campaign
The Phantine Skyborne first appeared in Dan Abnett's Guns of Tanith novel.