Grail war sounds pretty aight, my own complete unfamiliarity with the Fate series aside. +1 to that.
Well, the basic gist is that every player gets one Servant among their normal articles.
A Servant is effectively the spirit of a legendary historical, mythological, or on rare occasions fictional individual, manifested into a physical form(which has been limited from its original power by blocking out everything that doesn't fit with the Servant Class the spirit has been assigned) using the magical energy of their Master to maintain their body.
While Servants are generally encouraged towards obedience of their master because they get a wish if they win the Grail War(by killing the other participating Servants), they don't necessarily
have to be obedient if they disagree with the Master.
Thus, the Master gets three Command Spells which allow them to force the Servant to perform one action(the longer lasting or more vague the command the less potent the effect) each, even some things the Servant wouldn't normally be able to do. In fact, spending a Command Spell will actually empower the Servant in performing that action in addition to just forcing them to do it, provided you don't give a stupid command like 'win' or 'obey me'.
Servants have a set of skills(abilities which they can use to potentially best a Servant who is more capable than them otherwise), and one or more Noble Phantasms, which act as trump cards that can singlehandedly win a battle even against a notably stronger Servant if they don't have a good enough counter to it.
But a Noble Phantasm usually takes a lot of magical energy, and almost automatically gives away the Servants identity, which allows others to know that Servants strengths and weaknesses pretty much right away.
Edit: Also, on the topic of the prerolled sheets, there's only one that I would be particularly ruffled by getting rid of, so now that we've already had two people agree to get pre-rolled sheets, nobody should feel obligated to take them.
The only real difference between prerolled sheets and newly rolled ones is that there are wikis you could get in new ones that you can't in the prerolls since they weren't on the list then.