The Necron Crypt Stalker (Sometimes fan named as the Super Tomb Stalker) was a souped-up Tomb Spyder-shaped Necron construct on steroids that was unfortunately destroyed during the Medusa V campaign where a Chaos Titan blew it up before it could even be reactivated.
Judging from the description, it could be said that the Crypt Stalker is the largest known Necron construct and one of the largest Necron war machines ever encountered with the exception of Voidships. Although it could be smaller then either the Megalith or Abattoir, it could be much more faster, agile and swifter given its construct origins and the lack of any need to be stable in order to house a Necron Warrior. It is roughly unknown how powerful it is, although it will not be on the same scale as the sun-toasting rape machine known as the Æonic Orb.
The mere notion that Chaos needed a Titan to blow up a Necron construct fills one with imagination on how large and fucking powerful a Crypt Stalker really is. Many in /tg/ just think of the Crypt Stalker as that mythical and elusive Necron Titan that has been missing all these years while others think of it as a Spyder the size of a Titan. Unless GeeDubs or Forgeworld release a canon model of one with the obvious price of being more expensive than a real used car. We may never know.