Space Sharks

This is what some believe the Space Sharks look like.

Space Sharks/Carcharadon Astra
Battle Cry	None (They are silent during battle, though if they weren't it should probably be "I'm a shark, I'm a shark, suck my dick, I'm a shark!")
Founding	Either 2nd (according to a GW article) or 6th (according to FFG)
Successors of	Most likely Raven Guard
Successor Chapters	None
Chapter Master	Tyberos the Red Wake
Primarch	Probably Corax
Homeworld	Fleet Based
Strength	1000
Specialty	Operating out of the Imperium's Borders. Brutal close combat. Scorched earth policy.
Allegiance	Imperium
Colours	Grey, with black pauldrons and white helmet faces

This is what they really look like.

This is what happened when someone from Games Workshop watched Street Sharks and thought "there oughta be some spess mehrens like that." These guys call themselves Carcharadons" (Carcharadon, pronounced "car-care-uh-dawn" is the scientific name for a family of big sharks), or Carcharadon Astra if they are being extra formal, but everyone calls them by their low gothic name, the Space Sharks, because that is a fucking awesome name. They have light-grey skin and black eyes like sharks, and according to FFG they increase the resemblance by filing their teeth to be shark teeth. Their chapter logo is a white shark on its side and their colours are grey with black/dark grey shoulders. Once upon a time, they were light grey with a red stripe down the middle of their helmets and their logo was a stupid looking cartoon of a black shark with red eyes. Luckily, IA10 broomed that silliness under a rug and gave us what we have right now.

They're as mad as hell, even the Angry Marines might wanna think twice before cutting in front of them in line. They're described as sneaking into place before going batshit insane with killer rage. But, their rage is always controlled and stuff. So they're like Khorne Berzerkers mixed with the SAS. They also like to cover their armour with Polynesian looking tattoos. Also they try to stay quiet in combat, because they think it makes them coolsharks can't talk. Their specialty seems to be committing indiscriminate genocides. They also like to wear Heresy-era armour to give people more incentive to buy from Forge World.

The Carcharadons come up in the Badab War, where it's kinda hinted that they're Raven Guard mutants. Anywho, they're deployed to Afghanistan in space to fight the space Taliban and they solve the problem that the Fire Angels had been having by killing everybody to root out teh terrorists. Grimdark. Then they go over to Badab and blow the place up, ending the Badab War. Then they go back to the ashes of space Afghanistan and make a bunch of boys kill each other to become the next generation of Carcharadons before heading back out to smash someone else's planet. Also, according to rumor, they helped to trash Failbaddon's 7th Black Crusade.

Their leader is Tyberos the Red Wake, a terminator who couldn't decide if he liked lightning claws or chainfists better, so he made Lightning Chainclaws...IN SPAAAAAACE! He can rape just about anything in the game in close combat, and make his army swap combat tactics for furious charge and rage. He is reasonable enough to wear helmet and do not carry banner and shitton of flashy trinkets on his armour, unlike most other chapter masters, whose wargear just screams "Shoot me down, sniper!" Also, allows you to take assault terminators as troops and swap any boltgun in your army for a chainsword. And he can get preferred enemy for everything he kills too. Why aren't you taking him again?

Not as cool as the Saharduin, who are actually sharks from space. They do, however, share the same battle cry.

Their beakies look fucking amazing.

Alternatively, they're a Night Lords sucessor/loyalists. Bear with me here:

-Old as fuck and of no known founding

-Speak only High Gothic, ie what The Emprah was speaking

-Incredibly reclusive

-Pale with black within black within black eyes

-Use terror tactics (their new 6th Edition Chapter Tactics rule gives them Fear. The only Loyalist chapter tactics rule which does so)

-Lots of heresy era goodies (Their flagship the Nicor is a Charybdis-class cruiser, which was used by the Imperium during - you guessed it - the Horus Heresy)

-Has a leader who uses named twin lightning-claws like Curze did

-Suddenly appear, rush the enemy, rip and tear, and disappear

Basically, they fight like a more 'evil' version of the Raven Guard. The super formality and relic tech means they very well could have been in operation since the heresy and the fact that they patrol the edges of Imperial space means that there'd not be much of a chance for their cover to be blown. The palor and dark eyes are also markers of Night Lords gene seed, equally, they're described as having remarkably pure gene seed, which the Raven Guard does not.

Think about what you'd do with a bunch of loyalist Night Lords left over after the heresy. If you send them out to the edges, you don't have to deal with their creepy ass batmaness and if they turn on you due to crazy gene seed juju, they're not going to wreck your day. Equally, since they've proven their loyalty, their brutal stealth tactics are perfect for disrupting potential enemies behind enemy lines and this is exactly what Charchadons Astra do.

UPDATE: Charchadons Chapter tactics gives all Space Marines models Fear, and allows Tactical marines to replace their bolters with a chainsword or keep it and buy an additional close combat weapon for 1pt each. In addition they gain Rage after destroying or forcing an infantry unit to fallback in close combat, however they they must consolidate towards the nearest unit they can hurt (after all the rape train has no breaks!). You can only ally with Imperial Forces and all of those are bumped down to Desperate Allies.