Obelisk of Order Is a collection of massive crystalline formations that can be found in various locations in the Shivering Isles. These are usually inactive, but when they are active, the largest central crystal will rise and hover above the rest on a column of shock energy.
Active Obelisks will have a Priest of Order who guards it, along with the Knights of Order that spawn from the Obelisk. Kill the Knights and use their Heart of Order on the Obelisk three times, and it will then deactivate.
Remember that the Priest will revive him or herself a few times before one will be able to finish him. Also, if one try to loot the Priest's corpse, one will take shock damage, unless the Obelisk is inactive. The Priest, if still alive, can completely reactivate inert Obelisks of Order.