To answer the questions in the order posted;
@BCTheEntity I'm really glad someone asked.
A death-masque is a type of servitor. It is unique in two ways. Firstly, they are only made by the dark mechanicum. Secondly, the base creature used to create them is not human, but chaos spawn. Making them is difficult, but it can be done, and we just so happen to have the people necessary to pull it off. Though, actually doing it will be tricky. Which brings me to my next answer...
@Klomster Firstly, the hemlock is a bad WAR ship. For our purposes, it's nearly perfect. It sports one of the fastest engines in its size class (which is the class known for fast engines), packs a single weapon with Devastating capability, is purpose-built to be skeleton crewed, and best of all... The ENTIRE SHIP is made of warp-maliable material. Once we get it away from the eldar manning it, we can literally bend it to whatever shape we care to. All we'd need is a crew.
Which brings me to the second point, and this is where the death-masques come in. Urgrugg just so happens to know a spell for converting the bodies of mortals whose souls have been claimed by a single chaos god into chaos spawn-surprisingly, it's not that hard-while also restraining them. Any admech worth his salt would know how to make a servitor. With the chaos apothecary to help with the medical side of things, all we'd need is the materials to do the conversion. Namely, metal for the robot bits, and mortals to turn into chaos spawn.
Now, if only we had an ample supply of mortals whose souls have been claimed by a single chaos god... Oh, right. Space elves!
As for the metal... well, remember all the wraith bone I mentioned?
Before you ask, no, Urgrugg doesn't know how to manipulate it. However, he does know a certain tzeench daemon who just might. Yet another hand to help with the conversion process.
And, once we make our Eldar-flavor, wraith-bone, chaos-spawn servitors, we'll have our crew! With as many elves as we'll have, there should be no end of subjects to test on either.
As for the boarding... Well, our ship IS designed to be cloaked specifically from eldar craft. Add that to the fact hemlocks turn off all non-essential systems when they fire their gun, *cough*radar*cough*, and I think we can pull off getting aboard. Then, we just have to take the boat, and we'll be good to go. Which brings me to my final response;
@Wraithblade6 We're not boarding an imperial ship. We're boarding an eldar ship, which we're going to 'commandeer' the... everything, of. At which point, we'll all be aboard a chaos-dominated ship, crewed by a small slave army of death-masques directed by pink horrors.
And, before anyone asks how we'll take it; hemlocks are specifically NOT fitted to repel boarding action. Six space marines, a plague ogryn (gonna have to pull that one off quick), a magus, and a magic ork by themselves would be enough to give them trouble. When I start summoning daemons for back-up... Well, let's just say that's not gonna be ketchup on those khorn dogs.
So, everyone... are you ready to pilot a ship crewed by the damned and captained by daemons?
All in favor, say "Black Pearl!!!" Who's with me?