First off, I want to mention that this is an Apocalpse Dataslate, meaning its a formation for Apocalypse unlike the others we have seen recently. Centurions are one of my favorite units in the new Space Marine codex, so it was nice to see a formation for them. I do wish that it was not only for apocalypse.
The dataslate can be purchased by following the link, where you can also download a demo or preview the release.
http://www.blacklibrary.com/games-workshop-digital-editions
The formation consists of 2+ Centurion Assault Squads and 1+ Ironclad Dreadnought. All Centurions must be equipped with an omniscope. The formation itself comes with 2 special rules for the formation; Demolition Specialists and Seismic Devastation.
Demolition Specialists: While a sergeant is alive somewhere in this formation, re-roll armour penetration rolls, vs buildings, and can re-roll any glancing hits, but the second result must be accepted.
Seismic Devastation: If a building if the building suffers a partial collapse or worse from a close combat attack, those inside the building suffer d6 additional s6 hits that ignore cover.
Overall they fill a nice role on the apocalypse tabletop, but the limitations might become difficult if there are few or no buildings on the table. Once again, this is an apocalypse dataslate.