Well my idea was redo the scenario in the book as the middle of a campaign detailing Tostig's life like a Norse saga.
The tutorial scenario would feature his father during the initial waves of invasion before Arthur and Badon hill and have a scenario set in the treason of long knives where two players draw up Saxon bands (one Tostig's father the other Hengist's flunky) for there forces but both roll for each other on which of there forces will betray them and what actions the Celtics will do in the confusion.
It would then pick up in Tostig's youth during the middle to end of Arthur's reign and how he met his companions and learned his martial skill.
Starting with Tostig's exile and the circumstances that caused it, his father is slaughtered under trust at a audience by Ebusa youngest son of Hengist (a legitimised son beget by a germanian thrall)* after he demanded he convince the other thanes of the region to come to his banner against Arthur for his brother on the honour of King Hengist. Tostig's father:
"refused to raise banner to spend his people's lives for the corpse of a liar and a oath breaker who's honour is worth less than the mud tracked in by its emissaries and arrogant to assume they'd war for a phantom alone and offer not honeyed words of lands and legacy or even the insult of coin for the price of men. Though that itself raises the question, is he who sells his sword to such expected to bring long knives to the feast on victory? Arthur's line spared mine at Badon hill, he marches on holds beyond mortal men and Rome fears his name for it could be spoken in the triumph of a emperor one day, it fears yours as one does a chief born burglar, go tell your master come if he wishes alliance for when Arthur treats, he treats himself with the company of kings! not burden others with halfborn bastards and well broken mongols to spite his words from across a sea."
Ebusa and his mercenaries pulled weapons and set upon him, his head taken and lost, all men put to death, all boys greater then 6 summers to the sword, all other to the chains until the marriage of their mothers and sisters to men of the new lord and their sworn oaths before a folkmoot of Ebusa's rightful claim of the previous thane's holdings as wergeld for insult, broken oaths and the attempted murder of Lord Edusa and the reaffirmation of the guilt of said thane and his sons. the boy Tostig escaped as his father's last huscarl held his own guts in with a dinner plate while felling a berserker, Tostig dashing through the brush up the hill holding two generations of his ancestors looked out at his one last time, what was left of his father in the waking world now running hot in his veins and covering his back, uttering curses swearing on the embers of his home he would bring them home to Ebusa. Tostig fled to river a traced it long and far until the rocky outcrops was sand at his feet and the fields and trees before him became but waves and foam. Eventually he came upon a settlement with a jetty, half craven and half feral, caught by a seaman almost as wide as he was tall when trying to eat fish scraps, taken to market and about to be sold off his stare (and the seaman gagging him after several bites his favourite pleasure girl mistook for dog bites) arises the interest of a one (either Sheave The Surf or Fionn The Walker) a Celtic freebooter who figures "I've tamed worse, one leek and two onions?", he was got the reply "swap the leek for a carrot and its a deal", a bargain.
serving as a hand upon a crew of freebooters he experiences various episodes of danger and annoyance: raiding Picts and disturbed Romanians claiming to be Romans while being dragged into squabbles in Eire and Thule as mercenaries while trying to get as much out of it as possible, the strife of camlann (he can be on either side history only records he "fought like a beast made man"). To his dismay after the battle he lost track of the crew (he was look forward to a summer raiding Catalonia), after falling in with Peredur son of Efrawg and the now rather Falstaffian Dagonet enters a brief service as a bodyguard in the court of Justinian. Here under the pagan Tribonian (taught him ethics and how to exploit them and gave him his helmet which is horned as its a tribute to Pann) and Theodora's spy master Narses The Eunuch (teaches him about women). After getting separated from Belisarius (learned strategy and gained a obsession with using shields that "look Roman") and finds himself stranded in a cave in the mountains, he encounters echos of the Olympians and horrors colmenating in a battle with something claiming to be Pann and immune to mortal weapons, the player wins by strangling or biting out his neck screaming the name Woden (and Jesus), only to awaken from a dream but still holding the effects he'd torn from amazon and goddess and tasting a strange metallic and sulferius substance in his mouth reminding him (to a confusion that would remain till his last day) of venison.
That section would end with him being granted his title and showing how he became the outlawed thane we see in the game, having it so he gained Beowulf's life friendship* by slaying a wight what was set upon his uncle's people and fermenting a blood feud between two clans. King Hrethel grants him the title of thane and calls in a favour to call a folkmoot among the Saxon holds of Britannia to let his heritage be known and have his outlaw status be revoked as regardless of his father he has spent more than 20 years a outlaw longer than the sentence laid upon outlaws. Ebusa however finds out and using his influence to call the moot early allowing so not every representative can attend allowing him to get Tostig's outlaw-hood's expiration to occur in 2 years and a day. Hirelings are sent after Tostig and to harass Hrenthal's holdings, they are lead by a key huscarl of Ebusa's called Böðvarr who was amoug those who cut down Tostig's father, he is slain. Tostig not wishing to drag Hrenthal's people into a feud and giving Ebusa the chance to drag his reputation down and further canonise his lies, breaks ties and flees with his band after he is accused of murdering Böðvarr (despite it being a legal duel), he gives stewardship of his land to friend who builds a drinking hall upon it in his honor. First they go to Gaul and Hispania then to Brittany losing the thugs in Cornwall but they almost die in an ambush in Gwent (modern day Newport) that goes south when the local lord thinking its a raiding party sends his own forces in, scuppering the party's attempts to get to Eire undetected. Tostig's party is forced to stick to initially the Celtic regions of but the continued pursuit by Ebusa's men force them to have to risk the Saxon holds as incidents like the Gwent ambush cause more Saxon travellers to be viewed with greater surspesion and ironically make it much harder to hideout. After working as mercenaries for a spell they are offered a job to retrieve a lost crown to prove the legitimacy of a young impoverished chief son's claim to the rule of a hold so venture into a cave and stray into the domain of the Scuccas. This results in the party navigating the vast tunnels connecting ways alien to man's understanding alluding ambushes and the thugs foolhardy enough to follow them while trying keep there twigs of rowan tree to prevent time claiming them upon leaving. once out the Scuccas's king enraged at lose of his treasure invades the surface causing Tostig's party to take him out while the chief's son ties up his forces. This deed in league with others causes Tostig's reputation to increase and some hold leaders start trying to call a folkmoot early to pardon him, Ebusa realises the time before Tostig can challenge him without legal fallout is coming soon so seeds rumors of King Debobmik and the treasure of the sword Blooddrinker in whos it will be the end of him.
Then follows is a enhanced version of the original scenario greater detailing the characters and adding escape section as the domain slips into the mire. Ebusa insenced at Tostig's victory challenge's him early to try and prevent him from preparing, using his ties to Debobmik as a cause*. The duel is held at Tostig's home before witnesses, where Tostig finds what has become of it and that Ebusa forced his mother and then his sister to accept his hand in marriage. The duel is declared with magical assistance banned (Edith and the Blooddrinker) and Ebusa uses his son Torgeir by Tostig's mother, Böðvarr's younger brother and his young son Tollak by Tostig's sister as champions to try and exhuast, demoralise and force upon him the title of kinslayer, before it begins Tostiq's mother gives all partisapents a draft of spiced mead under the pretence of it being to wish all luck in the fight and grant the favour and approval of the gods. Tostig fights his oppents and upon stiking down Böðvarr's brother finds they are a disguised woman (horrorfying Tostig), he does the same to his half brother dispite both being reluctant to fight (Torgeir dives not wishing to slay the son his mother's waited a lifetime to see again) and to his disgust has to do the same to young Tollak in self defence due to him going berserk after see Torgeir lose. The elder preparesto call the deul and the matter in Tostig's favour but Tostig screams it is not over and challenges Ebusa again this time for the blood of his half brother and nepthew, Ebusa accepts believing he can now slay him. Tostig in a maddened rage holds his own and dispite rampent cheating from Ebusa inculding using a long knife in addition to his sword, in the end Tostig wins avenging his fallen family by beheading Ebusa and doing a similar speech to his to retake his holdings as wergeld. Tostig then turns to Beowulf and asks him to kill him and lead in his stead as he refuses life as a man like Ebusa, Tostiq's mother & sister and Edith stop him and take him to the priet's home to find his three oppenents are in critical condition but aren't dead as the draft they drank was a eilxair to slow the blood but increase pain preventing bleeding out and causing them to passout eaiser.
This would Lead to the Third section that would cover the sequel adventures covered in the original book with Torstig no longer a outlaw with his family holdings restored he seeks to to use his skills to gain glory and fame now no longer bound by revenge or need to survive. This section would have Torstig wielding the bloodrinker and made with scenario's being more like a combat puzzle to prevent it trivialising matters or making every encounter a christian. They battle Grendal, the giant goremagot, the scinnas and aid Merlin with each victory causing Torstig's reputation as a hero to grow along with his ambition and drive to seek greater hights as it goes to his head and the Blooddrinker curse to bring doom to it's barer starts to ring true. Eventually he begins thinking he can be a Saxon Arthur and starts anexing territory of petty lords and trying to get holds to follow him as a king all the while traveling the seas on voyages to hunt great monsters until his actions cause the stiring of the dragon Waerferth and meets his doom slaying it.
The final section would be a beastiary as well as a brief of Torstig's sons, their lives and that of Tollak and Torgeir giving fodder for custom scenarios that follow on and completing the effect of the book being a saga. There would be a larger section on the nature of the manuscript will I will ellaborate on later as I got delayed on this and had to shorten it.
*Ebusa LongKnife, the possible son or grandson of King Hengist ment as a cognate of the various confliecting accounts of his hiers as the saga's key and inciting antagonist.
*here being a earlier in life version of the legendary hero recast for the setting (similar how chivalric romance cycles traded characters and reimagined them, a good example being Bradamante of the Charlemagne cycle becoming Britomart of later Arthurian works). Here he'd be a slightly younger warrior who's father swore loyalty to Arthur and had aspirations of him becoming a knight even naming him a Saxon rendering of Arthur (they both mean bear) but with the strife of camlann Arthur's age is over and Britannia slips back into barbarism and the knights he squired for now hunted for their suits of armour. Landless, listless, unsworn and born for a world that could not live beyond the man who made it, never to know knighthood and cast to wander as a man of arms, contemplating if chivalry is to die so be it but shouldn't Saxon and Celtic not be better having seen it?
*Debobmik's the eton born son of Ebusa by a wood hag who grew and aged unnaturally fast and upon being granted a small territory to keep him out of sight he became king of the fen sprites, began worshipping symbols of entrophy and hunger as idols and dragging unsurspecting travellers into his domain.
