No Space Marine earns the mantle of Captain of the Chapter's 1st Company without the faith of his Chapter Master and the respect of his battle-brothers. Captain Karlaen has served the Blood Angels with distinction for over four centuries, ascending to the leadership of the Archangels after serving within the 1st Company for over a century of war and carnage. In his time he has gained the admiration of his peers and of Commander Dante himself. Arenos Karlaen possesses a keen tactical mind that has guided his brothers to victory time after time. It was this singular understanding of warfare that first drew the attention of his Company Captain and Commander Dante. While all other Space Marines undergo rigorous psycho-conditioning and constant tactical training, Arenos' abilities go beyond this. With a mere glance, he is able to gain the measure of a conflict and formulate an effective response, often drawing from the ancient teachings of the Codex Astartes as inspiration.
As a young battle-brother, Karlaen first displayed his tactical acumen during the war for Barestyr XIX against the Speed Freeks of WAAAGH! Kruggdak. When Karlaen's Sergeant was decapitated by an Ork Kannon, he assumed temporary command of his squad. Using the world's sky-bridge network as a battlefield, Karlaen trapped the xenos among the bridges' complex cogwork gates. Robbed of their mobility, the Orks became easy prey for the Space Marines and were quickly defeated. This action would be first of many to draw the eye of Dante. More victories followed, and with each one of Karlaen's renown among the Blood Angels increased. When Arenos entered the ranks of the Archangels, it was as a Veteran Sergeant of a Terminator Squad.
The destruction of more than half a dozen Space Hulks can be laid at Arenos' feet, and since entering the Blood Angels 1st Company he has led many of the missions to purge the Imperium of these insidious threats. Doom of Vorgoth, Divine Purgotory and Twilight Aegis are names now synonymous with Karlaen, as he was the Captain responsible for clearing these Space Hulks with only a handful of battle-brothers. It was on the Doom of Vorgoth that Arenos was almost killed by the Necrogolem, a Nurgle Daemon of meat and metal. The vile creature had turned the Space Hulk into a playground of pestilence and disease, filled with myriad traps to ensnare the unwary. Karlaen would eventually destroy the creature by venting its sump-palace to the void and freezing the Necrogolem in a tomb of its own filth.
Relic Terminator Armour - Tactical Dreadnought armour is among the finest personal protection available to the warriors of the Adeptus Astartes and it has saved Karlaen's life many times over. This suit of ancient Terminator armour is a mark of rank within the Archangels and a reward for his unyielding loyalty to the Chapter.
Aureate Halo - Karlaen's Iron Halo is a relic of the Archangels company armouries and is known as the Aureate Halo. The halo is reputed to have been crafted during the Second Founding of the Space Marine Chapters.
Hammer of Baal - Karlaen's Thunder Hammer is an ancient relic of the Blood Angels Chapter, given to him by Dante as a mark of his honoured place as Captain of the Archangels.
On the Tabletop
Finding rules for Karlaen is a little annoying, since he's inside the Deathstorm expansion for the Shield of Baal expansion (so you don't get him if you bought Shield of Baal box set), and even then he's mediocre at best.
His personal statline at first seems a bit...meh. Just a regular old captain in terminator armor with the bonus of an iron halo and a MC Thunder hammer and counter-attack. He can get the job done if used proper, but other named characters will most likely come out on top over him (looking at you Lysander!)
...And then you take a look at what he does to the Formation he gets attached to. +1 to seize the initiative rolls, in addition you can re-roll any reserve rolls, failed or successful. Holy balls! For a deep-striking army like the blood angels this is utter gold. Turning your entire force into an alpha striking juggernaut that can deep strike onto your enemy's molars on the first turn.
In the end, Karlaen, just like Krom Dragongaze before him, is a character you'll choose to play, because you like his fluff and model, and not so much for his rules. If you are out to forge the narrative and create some new quest for your favourite heroes, Karlaen, Krom and the rest of the newly introduced characters from the Stormclaw, and Deathstorm are your men. If you are out to win you some tournaments, leave him for some overpowered unfluffy bullshit.
In 8th Edition
The Hammer of Baal has become a standard Relic, so if you want to field this guy, just have a Terminator Captain wield it. Making for a pretty durable and killy melee fighter. Stock him with his normal storm-bolter and let him go. Not much fluff to go on as to what his warlord trait would be other than the codex saying that he's good at planning and he's usually pretty dour--sooo...I dunno...Soulwarden? Just roll for it, you really can't go wrong. Giving The Veritas Vitae to a Lieutenant could also be worth dipping another CP if you REEEEAALLY wanted to fluff it as his tactical acumen--I mean VV, it's always worth it, but this is for if you just want a fluff battle.