The Omega Protocol is the ultimate attrition tactic devised by the Imperium of Sidhae, which entails the complete destruction of a planet about to be overrun by enemy forces, permanently denying it's use to the enemy and causing massive casualties to the foe at no gain.
With the Loyalist side's victory in the Sidh Civil war in 2243, the reunited and weakened Imperium was faced with the onslaught of the human-Skargh coalition, the surviving Sidh forces being insufficient to hold back the invading juggernaut.
Imperial strategists were consequently hard-pressed to find effective means to even out the odds. After much deliberation, an idea was proposed by Grand Admiral Therion Vraak that had crossed the minds of many, but had thereto been unthinkable - the permanent destruction of a habitable world along with every enemy and surviving ally on it.
Although the Empress was no more comfortable with the idea than the rest of her subjects, the circumstances were dire enough to convince her to approve of such a step.
In the following two years, 28 Imperial worlds were obliterated to wear down the attacking human and Skargh forces. Although extremely costly, the strategy finally paid off in 2245 at Hades Gates, having worn down the bulk of the enemy force and stretched it's supply lines thin and vulnerable to Sidh raids by destroying strategically-valuable worlds along the way that could have provided fleet anchorage and supply manufacturing capacity otherwise.
Omega Protocol had not been enacted since the Age of War until the Altoris Campaign in late 2640's. After a massive Skargh invasion prompted the Imperium's neighboring Commonwealth of Altoris to request Sidh military aid, even the joint Imperium-Commonwealth forces proved insufficient to stop the Skargh advance. Increasingly desperate, the joint command under archistrategos Arcadius Drax proposed to grind down the invasion force through targeted application of Omega Protocol. After much resistence, the Commonwealth government eventually approved of the plan, nine Commonwealth planets being subjected to planned destruction, finally evening out the odds in what was the single largest military conflict the Imperium has had to face since the end of Reconquest Wars.
There are no specific methods of enacting Omega Protocol asides from that it must result in the planet becoming permanently uninhabitable. It can take the form of destabilizing planetary crust by detonating pre-placed powerful fusion bombs at tectonic weak spots, by commencing orbital bombardment targeting the same pre-determined weak spots with dreadnought-class particle lance weaponry, or, in principle, by any other conceivable means that would render future colonization and resource extraction inviable and result in immediate mass casualties of the invading enemy forces.
A recent initiative by Imperial citizens has requested that future battleships of the Imperium be named after worlds lost to the Omega Protocol during the Age of War. Thus far, this initiative has been left without a formal answer, though indications are that this may indeed become the case.
The concept of Omega Protocol is roughly similar and inspired by Exterminatus from Warhammer 40 000 lore. However, the methods of carrying it out are markedly different, Omega Protocol also being enacted far less frequently and arbitrarily than its 40k counterpart.