Avoid the use of "tape" - look for a less-obvious 40k equivalent. I would also suggest starting the Acolytes out simply investigating some mysterious deaths and looking for a cause. The common thread between the dead people would be a dangerous interest in items of mystical significance and, eventually the Acolytes discover, membership of an informal circle of traders in such things, scattered across the Calixis Sector, further members of whom appear to be dying under mysterious circumstances.
About this time, rumours of a "special" hololith that confers secret knowledge and enlightenment should surface. If at least one Acolytes can be persuaded that the solution to some mystery or tragedy in their past might be solved by viewing the hololith, so much the better!
The Acolytes will need to infiltrate the circle and track down whoever keeps moving the hololith (pick an antagonist, any antagonist...). If none of the Acolytes themselves want to view the hololith, give them a valuable ally who secretly wants it for him/herself so that, once it is tracked down, their ally will turn on the Acolytes to seize the hololith.
Obviously this person - or the relevant Acolyte(s) - will view the hololith, thus releasing the tormented soul of the Eldar who has been trapped on the hololith (diverted from his proper path to his soulstone by some idiot who will probably turn out to be the Acolytes' master or antagonist [or both!]) and who has now been reduced to nothing more than a beast desperate to possess true flesh once more.
Cue mad fight in which the Eldar soul possesses one Acolyte after another, turning them on themselves. At a crucial moment it will possess an NPC. You should have, by this point, dropped in some other item they can use to destroy the soul, banish it to the Warp or trap it in its proper place: perhaps they will have to choose between several items with several possible outcomes. If they can trap the soul in a soulstone and return it to the Eldar craftworld it came from, they will have powerful allies (but be under suspicion within the Ordo Xenos); if they make use of alien tech to eliminate the soul they will likewise have crossed a line. If they use Imperial tech - such as a psycannon - then you could have a whole side-adventure seeking to steal such a precious item from the Ordo Malleus, pending future troubles...