Some creatures are so massive that their sheer bulk is a threat all of its own. Particularly massive monsters have an altogether devastating stomp.
Source: Warhammer Armies Project: Unofficial 9th Edition (v2.2)
StompURL Copied!
A model with this special rule can make a Stomp in addition to its other close combat attacks (including Breath Weapons). A Stomp has the Always Strikes Last special rule, and inflicts D6 (or D3 against lone Infantry models and skirmishers) automatic hits, at the model's Strength, on one enemy Infantry, War Beasts or Swarm unit in base contact with the model and are randomised as Automatic Hits. This represents the creature crushing the foe beneath its ponderous feet, or knocking their broken bodies aside with one sweep of its mighty tail (probably while it's roaring in a most intimidating fashion).
Unless specified, any special rules that apply to the model's normal attacks do not apply to its Stomps.