Hivefleet Gorgon lore

8th ed codex [Tyranid]:

"FIRST ENCOUNTERS

The T’au Empire first encounters Hive Fleet Gorgon upon the forest world of Sha’draig. Initial T’au victories swiftly give way to attrition, as the Tyranids adapt to counter their opponent’s every weapon and tactic. The Empire desperately rushes experimental rail weapons and prototype macro-missiles to the front lines. For a brief time, it appears that these new technologies will prevail. This optimism lasts until Gorgon attacks again. This time, the Tyranids seed the skies above Sha’draig with clouds of choking spores. Hundreds of Fire Warriors collapse in frothing seizures as the spores clog their respirators. Mawlocs burst forth beneath the T’au defences, crushing the stricken defenders in avalanches of rubble. Those few T’au left alive rush to evacuate the planet. Sha’draig is devoured."