"Swallow's take on Seth is a guy who understands exactly what he is and will play his given role to the hilt, and damn the pitfalls. He consistently strikes a balance between acting like a complete dick and yet somehow hitting the precise level of dick that he gets you steamed while also garnering exactly the attention he needs to get his point across. It's like he's attained a zen level of dick other dicks could only hope to emulate."
Father of the Flesh Tearers
When Seth became the Chapter Master of the Flesh Tearers in 815.M41 he immediately was told bad news by his Chapter's Sanguinary High Priest. The regular attrition on the forces of the Flesh Tearers combined with the degree the Chapter lost its members to the Black Rage the Chapter could no longer recruit enough new Marines to keep replacing losses: the Flesh Tearers were going extinct. Seth did not take well to this, but he vowed that if the Flesh Tearers were to burn up they were not going to smolder out, oh no. If the galaxy was to lose the Flesh Tearers it would do so in a massive conflagation fueled by the bodies of heretics and aliens, mutants and daemons with the massive roll of honor of the Flesh Tearers as the capstone to the grave. And so started the epoch of Gabriel Seth and the Flesh Tearers (which is an excellent band name by the way).
Seth knew that his men were all but frothing berserkers who slaughter anything and everything they run into on the battlefield, be they friend or foe. As such many Imperial Guard and Adepta Sororitas commanders dislike fighting alongside the Flesh Tearers: they were rather likely to turn on their allies and civilians alike (perhaps as some kind of proto-Khornate Knights?) and RIP AND TEAR through them. The Chapter Master decided to aim the zeal towards enemies with less non-targets around: diving head-first into dangerous situations in enemy territories and passing distress calls along to other Chapters who do not butcher entire planets. Thus far Seth's plan is working: those who were out for the Flesh Tearers' blood at first are now praising them, though (of course) some skeptics remain.
Son of Sanguinius
The Flesh Tearers are a Second Founding of the Blood Angels and as such hold the Chapter as first amongst peers. When the Blood Angels called for a conclave of the successor chapters. Seth was at first reluctant to answer, but he quickly figured out he could gain from the situation. When he met with the others as the conclave (and a nearly lethal duel between two Brother-Sergeants of the Blood Angels and Flesh Tearers) Dante revealed that after a big internal conflict (not as big as the Blood Ravens civil war, but damn close) the Blood Angels had lost many of their men. So Dante had decided on two things: to accelerate the Blood Angel recruitment process and that all of the successors were to pay a tithe of Space Marines to their progenitor. Seth told Dante where he could stick it, then went even beyond that by suggesting Dante did the reverse: disband the Blood Angels and distribute their Marines and wargear between the Second Founding Chapters.
Before Dante could relieve Seth of most of his primary and redundant organs a subordinate of the Blood Angels High Sanguinary Priest, Caecus, claimed to have found a solution: he had managed to artificially age a recruit into a fully-able Marine in a matter of weeks. Dante was not amused, but Seth was eager to see if the Priest's claim was true, with the future of his own Chapter in mind. Unfortunately the recruit (called a Bloodchild) was as flawed as the last time the aging techniques were used by Corax, and the Bloodchild succumbed to the Black Rage in the middle of a combat trial and had to be put down. This infuriated Dante and ordered the research on the Bloodchild destroyed. Seth took the chance to approach Dante and made a bold offer: to disband the Blood Angels entirely and make them all Flesh Tearers (an offer that's retroactively hilarious when it's revealed in a novel that decades ago Dante passed down a verdict for the Flesh Tearers to be disbanded and absorbed into the various Blood Angels chapters). Dante told Seth to go fuck himself, but then the alarms went off. Caecus had in a bout of desperation and an offer by one Magos Serpens made a terrible mistake: added blood from the Red Grail to the mix of the Bloodchild's prototypes, which spawned a horde of monstrosities called the Bloodfiends. With a nice trollface Serpens then revealed himself as Fabius Bile and the Bloodfiends broke loose. They tried to defile the tomb of Sanguinius, but with the help of Gabriel Seth and the other Second Founding Chapters the Bloodfiends were destroyed. Seth himself held the Priest Gate with Dante (postmortem analysis by Sanguinary High Priest Corbulo revealed that one in five of the Bloodfiends killed at the Priest Gate died of injuries resulting from a shattered pelvic bone), and Dante finally let out his frustrations, saying that Seth thrived on disorder. Seth put on a huge trollface and admitted as much, saying that he was essentially Dante's watchman: while the Flesh Tearers were the Great Angel's battle savagery, the Blood Angels represented all of Sanguinius and had to be held to a higher standard. Dante also proved that there was no bad blood (no pun intended) between two Chapter Masters when he rescued Seth from a Bloodfiend, calling him "brother" instead of the normal "cousin". Bile escaped with some of the blood of Sanguinius and became a target of the Blood Angels from that day on.
With the Blood Angels more, well, bloodied than ever, Dante repeated his request but added that it was nothing more than that: a request. Seth was the first to step up and repeated the final line of the Flesh Tearers Invocation Initiate to prove his point: if you messed with any of the Sons of Sanguinius, you messed with all of them, and pledged some of his men to the Blood Angels.
As of the current state of the Imperium, someone high up somewhere apparently wants Seth dead. The Chapter Master was lured to an old battlefield to make a vigil for his fallen brothers, but was trapped in an armourglass cell with an Eversor Assassin courtesy of a traitor (not that kind) Captain from his own chapter, who presented an Inquisitorial rosette and the alleged fact that Seth was leading the chapter to ruin as his justification. After being stabbed through the heart and in the forehead; Seth only barely managed to defeat the Assassin, pointedly NOT killing it by smashing the Eversor to a bloody pulp and dragging its still living heap of a body back to his ship, where he locked it in a room with his former captain as a just punishment for the betrayal.
The Devastation of Baal
Gabriel was one of those who heeded Dante's call to all Chapters of the Blood to defend the Blood Angels home world from the incoming tendril of Hive Fleet Leviathan. In a move that surprised Seth himself, Dante called him friend, and privately loaned the Flesh Tearers a priceless treasure -- the Reliquary of Amit, the only work of craftsmanship done by the Chapter's first Chapter Master. In it was held the last remaining feather of the Primarch, preserved for all time.
The Flesh Tearers were assigned the defense of Baal Primus. When Seth found out that the renegade Knights of Blood had also decided to defend the area, he had to try long and hard not to pick a fight with his erstwhile "cousins", specifically the Knights of Blood Chapter Master Sentor Jool. After a slight detour and a history lesson, Seth decided to leave the Knights of Blood alone, but left an implied threat that it would not be a good idea for the two Chapters to meet on the field of battle.
The arrival of the Tyranids made any confrontation between chapters moot, and Seth threw himself into the defense with gusto, tempering the strong urge to Rip and Tear with actual tactical thought. The defense wore on, and the Chapter casualties mounted, and Seth would not expend his forces needlessly. This surprisingly strategic turn of thought enraged the Chapter's last remaining Reclusiarch, Appollus, who thought it would be better to lead the remaining Flesh Tearers into the horde and die in a blaze of glory, rather lose marine after marine in an unglorious defense far from Baal.
Faced between a quick if glorious end for his Chapter, and a slow one, Seth proverbially said to himself, "Screw this!" -- if the Flesh Tearers were to die out anyway, it would be doing something important. He decided to withdraw his forces to Baal, but not before dragging the Knights of Blood with him. He fought his way to where the renegades were holding out, deep in the Tyranid horde, and "convinced" Sentor Jool and his men to follow him back.
Then the Fall of Cadia happened.
The Great Rift grew, and with this (and the failure of a ritual on Baal by Mephiston and his coven of librarians from many Chapters of the Blood) the skein of reality grew thin. Seeing its chance, Ka'bandha, the Angel's Bane, manifested in orbit around Baal Primus, and fell like a Warp-spawned meteor upon the moon.
The Angel's Bane
Brawler on the Battlefield
"...indeed he himself is prone to rage-soaked savagery as any of his brethren."
- – Blood Angels Codex
Gabriel Seth foregoes the traditional Power Weapon of his fellow Chapter Masters in favor of the Blood Reaver: a massive sod-off Chainsword that in crunch terms doubles his strength to a total of 8 and gives a somewhat lacking AP-2, which funnily enough means that in 8th Edition he deals more damage than ever. It always deals 3 damage, and all rolls to hit of 6+ automatically inflict another hit. And while Seth has WS 2+ (with rerolls) and 4 attacks, well-armored foes might prove a problem. But if he manages to wound them all but the toughest targets die immediately. This also means that he's surprisingly effective against light vehicles, especially when backed by a properly-geared squad. Do not forget that he also has a Bolt Pistol and Frag/Krag grenades, which can really help out in a bind. He also has the Lord of Slaughter ability; if a squad finishes its consolidation within 6" of him it can immediately fight a second (but not a third) time, meaning that if Seth's unit does not immediately wipe out its opponent it has a second shot at doing so. And at 135 points (7 Power) he is a rather good purchase as long as you also have a dedicated assault unit for him to join (Company Veterans and Vanguard Veterans without Jump Packs come to mind) and a matching transport.