Celano Fragments

What is 'X' again? confused as to what I'm paying X for and what X cost of the character would I be removing? I thought unless stated 'X' was zero?

When you play the card, you set X, which can be equal or less than the value of the domain being used to pay for the card.  When the cost of the card is referenced at any other time (not when it's being played) the value of X=0.

So if you pay 3 (for example) then you can remove a character of cost 3 or less from a story.

this works better in a Misk deck rather than a syndicate deck?

Well, you would need to have a Miskatonic resource on your domain in order to play it, so you have to have at least SOME Miskatonic in the deck (or a way to get around that).  Once it's in play, you can use resources of any type to pay for it's ability, so you could probably do fine with just a small splash of Miskatonic in a mostly-Syndicate deck if you wanted.

However, if you wanted to do that you might take a look at some other cards which may do the job more efficiently.  Seventh House on the Left is a Syndicate card that can exhaust someone each turn.  That's not quite as good as uncommitting (because they know before committing who is out) but it doesn't require another faction and has no cost to activate it.  As a more drop-in sort of replacement, you can look at The Cavern of Flame.  This is a Hastur card, only costs 2 instead of 3, and has no cost to use it.  It's slightly conditional (your opponent could have Dreamlands locations too) but mostly I'd consider it an upgrade to Celano Fragments.  However, it does depend on whether you'd rather have Miskatonic or Hastur as a 2nd faction in your deck, and of course the Hastur box isn't out yet.