A Graviton Ruinator deployed on a Warlord-class Titan.
A Graviton Ruinator is a Titan-grade Grav Weapon deployed on Warlord-class Titans during the Great Crusade and Horus Heresy eras that unleashes a directed energy particle beam of gravitons towards a target that dramatically increases the local gravity, crushing the foe under its own mass. It possesses a longer range and more stopping power than its smaller Titan-grade counterpart, the Graviton Destructor.
Graviton weapons are a rare sight on the battlefields of the 41st Millennium, mostly handheld, used to crumple heavy infantry and light vehicles by turning their own mass against them. Turn one of these present-day Grav-Guns onto a large walker or even a Titan, though, and it will do little more than bubble the paint.
During the Horus Heresy, however, things were rather different. Arcane, gravity-based weaponry was everywhere -- and it was a whole lot bigger and more powerful. Big enough, in fact, to fit on a Titan. And definitely big enough to destroy one.