Phyrexian Obliterator makes dealing damage to your creature an expensive proposition. Whenever a source damages it, that source's controller sacrifices that many permanents. A blocker that deals several damage can therefore cost its controller lands, creatures, or other resources even if the Obliterator dies in the exchange.
The trigger counts damage actually dealt, not the power of the source or the damage an effect originally attempted. Destroying or exiling the Obliterator without dealing damage avoids that trigger. This makes it a deterrent against combat damage and burn rather than universal protection against removal. Opponents may choose not to block it, allowing trample and its substantial body to pressure life totals.
Fight effects offer a more deliberate way to exploit the ability in a deck that can support another color. Having an opposing creature deal damage to the Obliterator can force that creature's controller to sacrifice permanents. Check which creature survives and how much damage will actually be dealt before choosing the target; a large damage amount can still kill your Obliterator even while creating the trigger.
Four required black mana is the major construction constraint. Mono-black decks support it comfortably, while multicolor builds need a carefully planned mana base. Devotion strategies also appreciate the four black symbols. The card works best when it changes combat decisions and forces opponents toward narrower answers, giving your other threats or engines more room while they decide how to handle a creature they would rather not damage.
