Plasma Obliterator Platform
The weapon itself is an enormous plasma weapon sitting atop a reinforced pillbox, the Plasma Obliterator is a magnificent piece of Adeptus Mechanicus war technology that could essentially turn superheavies into slag and force Titans to re-roll for critical existence failure.
Unfortunately, the fluff didn't really translate well for the crunch. This thing is hilariously bad. On the surface, it doesn't sound like it should be: a nice big gun, a lot of wounds and toughness, reasonable enough cost for both.
However, it has no safe firing mode for its gun and has a BS of 5+. This means that it wounds itself once for every two hits it scores against the enemy. All for the firepower that a single tactical marine with a plasma cannon could give. Seriously, you would expect something of that size to hit way above its weight right? On top of all this, it's somehow cheaper than the Bastion while keeping the same transport capacity and firing points.