I would like to share my idea about BC adventure and hopefully get some tips before I actually start GMing people through it.
A system-wide chaos invasion in Imperial space led by powerful and cunning sorceror was going exceptionally well - loyalist forces surrendering, defecting and getting massacred on all fronts, the capital spire turned into hopeless last stand ground by surrounded remnants of astartes, guardsmen, sororitas and civilians with nothing more than a basic training, a gun and unwillingness to admit defeat.
And then something which the sorceror failed to foresaw happened - mortally wounded sister of battle miraculously came back to life as Living Saint, unkillable heretic-purging machine. For chaos forces this is a problem that must be dealt with before slaves of the corpse-emperor regroup themselves - and that's when PCs, one of the warbands the sorceror have already used for delicate tasks appear on stage to fight back the unexpected foe.
It quckly becomes obvious that Living Saint is invulnerable to regular weapons - she can barely notice heavy bolter fire and shrugs off artillery shells. Direct macrocannon hit from Iconoclast destroyer orbiting the battle? She resurrects in a matter of seconds, and this bolsters loyalist's morale even more.
Sooner or later, whether from divination, forbidden tomes or common reason, PCs realise that the only way to deal with the problem is to use the trick servants of the Dark Gods used through the whole Long War - corruption. Forcing the Living Saint to assault holy imperial shrine secured intact by warband, capturing and using her fellow sisters as meatshilds when facing her and every single dirty trick the PCs can come up with adds up to Living Saint's Dark Heresy-style corruption track. When the track reaches 100 mark - she is no longer a Living Saint, and where she once stood, a sister of battle bleeds to her death, days before what's left of imperial resistance is overwhelmed.