I think it completely suffices if nuclear weapons (fission and fusion) remain the strongest weapon in existence. After all, one need not obliterate an entire planet just to obliterate anything on its surface. Heck, even the Empire in Warhammer 40k, known for its excessive use of destructive power, performs an Exterminatus by carpet-nuclear-bombing the surface, for example.
And then, since ED wants to remain mostly realistic, a weapon to unleash enough energy in a sufficiently short amount of time, to disintegrate an entire planet, would likely be so massive, that the mere act of moving it around would cripple the resources of its owner.
If there will be a singular doomsday device in ED, as opposed to just a massive attack with a lot of nuclear weapons, it might come in the form of a huge engine strapped to a large asteroid (like the one that killed the dinosaurs), bringing it on a collision course with the target planet. Still a massive undertaking, but probably doable for a large political entity willing to expend the resources.