Xiphon pattern interceptor
First brought into Imperial service in 806.M30 from the Rhadamanthys Enclave and used by the Legiones Astartes during the Great Crusade and Horus Heresy, the Xiphon Pattern Interceptor was a fighter aircraft based on designs so ancient its origins were long lost even in the Crusade/Heresy era (though it could be presumed to date back to when humanity fought the Cylons Men of Iron). Capable of both atmospheric and space flight, the Xiphon carried two pairs of twin-linked Lascannons and a Xiphon Rotary Missile Launcher into battle, allowing it to destroy both air and ground targets with ease. This firepower, combined with the Xiphon's unparalleled speed and maneuverability, made it a deadly opponent. The problem was, the Xiphon was complex, had limited range, and was ill-favored by the Mechanicum. Astartes were also pretty much the only ones that could fly the Xiphon effectively, as the fighter's performance put great stress on its pilot that few unaugmented humans could withstand (how anyone flew it before the Astartes came along is anyone's guess). By the end of the Great Crusade the Xiphon had largely fallen out of frontline service in all Space Marine Legions except the Dark Angels, Ultramarines and Thousand Sons, though the demands of the Heresy forced many Legions to press them into service once more. The White Scars made good use of them during their hit-and-run campaign against Horus' advance. No word at all on how many supposedly survived to 40k, but like many Heresy era stuff, if there's 40k rules for it, then it can be assumed that someone's still got a few squirreled away.
For those that want to field a Xiphon without paying Forge World, a Dark Angels Nephilim Jetfighter can be easily converted to fit the bill most satisfactorily, if having the Battlestar Galactica jet isn’t that important.
The Xiphon is named after the "Xiphos", an Iron Age shortsword used by the Ancient Greeks. The name Xiphos apparently means "penetrating light" according to researcher and swordsmith Peter Johnsson - which is very appropriate for something that's basically a flying Predator Annihilator.
Tabletop
The Xiphon Pattern Interceptor is a flyer that is available to all Legiones Astartes armies in Horus Heresy games. In 40k the Xiphon is available to Space Marines, Space Wolves, Dark Angels, Blood Angels, and Grey Knights, but only as an experimental rules PDF.
Horus Heresy (30k)
Now this is why people think twice when they hear "Forgeworld rules." It will also make you think twice when taking Lightnings because this is essentially a souped up Lightning. Comes with Two Twin-linked Lascannons and a Xiphon rotary missile launcher - Heavy 2 S8 AP2 that re-rolls successful Jink and cover saves and rolls d3 times per Penetrating hit, choosing the best one (and AP grants +1 on that table). All that makes this your flying tank killer. AV11 all around, this murderous potential is 'balanced' by costing about as much as a kitted-out Lightning and being extremely limited in its optional wargear: Chaff launchers and Extra Armour for survival and Ground Tracking Auguries to kill Land Raiders after you've cleaned the skies. And we say balanced because it comes already equipped with Armoured Ceramite, and Agile grants it a 3+ jink save, enjoy!
40k
It’s exactly what you'd expect an interceptor to be: light, fast and good against other vehicles, be they tanks or other flyers. Comes with two twin linked lascannons and a rotary missile launcher. The launcher is two shot, S8 AP2 and forces jink and cover saves to be rerolled, plus when penetrating hits are rolled, it chooses the best of D3 results (adding one for AP2). The Xiphon is a great vehicle hunter, but as mentioned its light. Only two hull points mean it'll go down against a stiff breeze, but with AV11 at least it's immune to small arms bolter fire just like a Stormtalon. If you want a good all-rounder you should take the talon which at least has a hover mode, but this thing is a dedicated vehicle killer.
8th Edition
The Xiphon remains a highly competitive flyer and armor-killer, as the changes to twin-linked rules mean its lascannons now put out 4 shots instead of only 2. Unlike most flyers the Xiphon also ignores the penalty for moving and shooting heavy weapons (and being a flyer it has to move a minimum distance every turn), on top of getting +1 to hit against models with the FLY keyword - which is a lot (Raiders, Battlesuits, Wave Serpents, etc).
The missile battery is now: Heavy 3, S6, AP-2, and does an incredible flat 3 damage. Perfect for taking out the lightest of light vehicles...or one-shotting Adeptus Custodes. Alternatively, one can see it as a 3-shot Krak Grenade with buffed damage.
The only downsides are that; A - the Xiphon can't take Chapter Tactics, B - you are limited to only taking 3 of them in a typical 2K game, and C - Boots on the Ground, a rule for Matched Play that essentially means if you have nothing left on the board except Flyers and empty Fortifications you're instantly tabled.
Death From The Skies Update
With GW's release of Death From The Skies (DFTS) Forge World have released a doc of updates for their non-Codex flyers (currently here: https://www.forgeworld.co.uk/resources/fw_site/fw_pdfs/Warhammer_40000/Death-from-the-Skies.pdf). These rules go a fair way to forcing the BSG Viper...er...Xiphon into it's designated Interceptor role, with the Sky Assassins special rule locking Zooming Interceptor-types into having Skyfire active and the Xiphon, not having the (Hover) sub-type, is always Zooming.
As Skyfire is always active the Xiphon is reduced to firing snap shots at any ground vehicle which isn't a Skimmer, meaning that if you take the Ground-tracking Auguries option you only get Strafing Run's +1BS against Skimmers as specified in the Skyfire USR. Along with Skimmers, Jetbikes, Flyers, FMCs & GMCs are still legit targets with Skimmers & Flyers being quite fuckered by the Xiphon RML's Cluster Warhead rule.
On the flipside, the update gives the Xiphon a Pursuit of 5, meaning it can shift an extra 5" when going Flat Out and combined with Supersonic this tops the Xiphon out at a max move of 41" in a single turn making it the fastest Flyer the Astartes have. The Xiphon is granted an Agility of 4 so it can perform a second 90 degree turn at any point during it's movement on the roll of a 4 or less. Not bad at all; only the Tempest & Eldar Flyers have better.
"LAWLS, WE DON'T PLAY WITH CRAPPY DFTS RULES, PSYCH!" you may say but as an observational note it's worth considering that core rules from both Escalation & Stronghold Assault were rolled into the 7th edition (current) 40k BRB so for 8th edition all bets are currently off but you never know... And then 8e came along and made it a flyer interceptor as a main role (but at least it can shoot normally at ground targets again).