The T'au discovered Ag'Ni very soon after they arrived in the Reach, long range sensor scans suggesting the world would prove ideal for early settlement. In fact, the world appeared so ideally suited for colonisation that a Water Caste contact cadre was assembled in order to bring the word of the Greater Good to whatever natives lived there.
The T'au envoys were greatly surprised to discover that the world was entirely untouched by intelligent life, though its continents and island chains teemed with plant and animal life.
It was only when the first of the Water Caste envoys removed the helmet of his sealed environmental suit and took a deep breath of the seemingly pristine air that the T'au discovered why no higher life forms had settled Ag'Ni.
The instant the envoy breathed the air, his system was flooded with airborne microbes so voracious that he was dead before his body hit the ground.
Within seconds, his horrified companions bore witness to his body being broken down before their very eyes by the most aggressive microbial life forms the T'au had ever encountered.
Contact doctrine dictated that the surviving members of the envoy team should be quarantined on Ag'Ni, and supplies were dropped to them from orbit. Yet, within a solar day the remainder of the team was dead, the microbes having breached their suits and overrun their systems.
The leadership of the Velk'Han Sept has since declared that Ag'Ni should be purged of all life so that it can be reseeded by Earth Caste geo-engineering cadres. This process is still underway, and the planet has come to be used as a weapons testing ground by the Fire Caste, who have been given free reign to test the most destructive of devices without fear of unleashing unwanted levels of destruction.
Every form of weapons technology has been tested on Ag'Ni, from planetary defoliant to ionospheric inversion. Mass yield pulse plasma warheads have levelled entire mountain ranges and the detonation of a seismic discontinuity charge brought about the creation of an entire chain of super-volcanoes at the world's southern pole.
Despite the destruction wrought upon Ag'Ni, the voracious microbial predators continue to exist. In fact, even in regions subjected to high levels of radiation the microbes persist, recolonising the blasted wastes within solar days.
The proud Fire Caste has begun to take the eradication of the microbes as a matter of honour, and the world has taken on the character of a warzone in which a bitter campaign is being fought against a totally invisible enemy.
The Earth Caste geo-engineering cadres have been side-lined by the Fire Caste and there appears to be no end in sight to what has long since escalated into a bitter war of conquest against an enemy that, it appears, cannot be beaten by even the most unconventional means.
- Deathwatch: The Achilus Assault (RPG), pg. 114