>>96836960
Still working that part out myself, so far I have two ideas
>Loyalist route
A splinter company of NL’s took such a thrashing that they saw the writing on the wall; being a traitor isn't very sustainable. When they found a holdout of WS’s instead of turning their guns on them they lent them their shoulder in hopes to get back into the good graces of the Imperium. Fast forward a few months of fighting and they’re all practically brothers in arms
>Traitor route
Isolated from their legion, a WS company with prominent terran-born marines in leadership positions let their true loyalties show by asserting their allegiance to Horus. Carving out their own little warpath, they eventually find themselves meeting their match, only to be aided last second by a NL company. Never one to waste time trying to recuperate forces, the WS’s blazed on with their new NL allies in tandem, reeking havoc across systems in mindless pillaging and massacring