Fan-made material. Be advised that the following is non-Canonical in nature.
Tel'Oshi is a Sept World located in the south-western region of Tau space. It is in the centre of the Tel'Oshi Cluster, and in recent years has seen repeated conflict against the Tyranid race.
Tel'Oshi was assaulted several times during the 41st Millennium by Hive Fleet Kraken. The initial assault landed on the planet itself, though it was eventually repelled. Smaller splinter-fleets were subsequently detected, and Tel'Oshi Cadres were deployed against them.
These actions gave the Tel'Oshi a reputation as Tyranid-Hunters, and many of the most experienced anti-Tyranid forces in the Tau included at least one squad of Tel'Oshi Firewarriors.
In answer to the invasions, the Kor erected a 'Dive Net' beyond Tau space. This 'net' was a series of automated Messenger Drones that periodically signalled themselves to the next link in the chain, relaying information about any objects within sensor range. The Tau soon learned that, prior to a Tyranid attack, the Drones would cease transmitting or be scattered from their anchorage. As such, the Tau had time to prepare themselves for subsequent Tyranid attacks.
The Coming of Kraken
Shortly after the Dive Net was established, a significant Tyranid fleet was detected. Far larger than any previously encountered, the Tel'Oshian assessments indicated that if the Tyranid fleet were allowed to consume Tel'Oshi and its surrounding worlds, they would have sufficient biomass to storm into the Empire's First Phase region, and annihilate the Empire.
Under the supreme command of Shas'O Tel'Oshi Li'Sun'yi, the Tau assembled on the seven planets within the Tel'Oshi cluster determined as the most probable targets of a Tyranid attack. Stretched to breaking point, O'Sun'yi personally led the fighting on Tel'Oshi. The ground forces had but one mission; stall for as long as possible. O'Sun'yi knew it would be the fleet that won the campaign, and her belief was ultimately proven. Despite horrific losses in both the orbital battle and the ground campaign, the Tyranids were repelled.
Each of the seven worlds of the conflict were left with lasting scars.
Jel'yth was reduced to barren rock, stripped of all bio-matter. Fortunately, the Tyranid Fleet was caught during its breeding cycle, and exterminated before it could rebuild itself. Tel'Oshi recovered well, albeit with minor alterations to her ecology due to mycetic spore attacks. Pral'kosh was transformed into a tropical planet filled with carniverous lifeforms. Kin'os was so badly altered the Tau were forced to subject the world to orbital bombardment, and purge all life; the threat of lasting Tyranid infestation was simply too great.
The Shas in particular suffered horrific losses during the war; 90% of all Shas deployed in the fighting were either killed or critically injured.
Shas'O Li'Sun'yi herself was left with lingering paralysis, although she would subsequently recover.
The conflict also saw the loss of Aun'O Tel'Oshi Shia, one of the most beloved Aun of the region. His body was mutilated beyond recovery, and was buried in a mass grave.
Operation Final Hour
When word of another Tyranid attack reached the Shas'Ar'Tol, they feared the worst. Tel'Oshi had not yet recovered from its previous conflict, and it was believed they would be unable to withstand another attack.
Fortunately, the Imperium of Man recognised the threat the Tyranids represented. They knew if the Tau Empire were consumed, the resulting Hive Fleet could potentially destroy most of the Eastern Fringe. Desiring to destroy the threat before it was realised, a large Crusade force, including an entire Chapter of Space Marines, was dispatched to fight beside the Tau.
Codenamed "Operation Final Hour" by the Imperials, the Tyranids were engaged on Tel'Oshi once again. This time, however, their strength was too great. Both sides were forced to evacuate, leaving countless dead and wounded in their wake. The Imperials subjected the world to Exterminatus to deny the Tyranids bio-matter, and the alliance regrouped.
In the confusion of the withdrawl, a previously unknown Chaos Sorceror known as Tzanoth made a move against the Tau. Seeking to try and corrupt the Tau to the service of the Dark Gods, Tzanoth was able to destroy several minor installations on the rear line, as well as capture O'Sun'yi herself. Despite losing their leader, the Tel'Oshi Cadres and the Imperium of Man were able to halt the Tyranid advance, and ultimately reclaim Tel'Oshi. The Tau instigated a terraforming program immediately.
O'Sun'yi was eventually found and rescued, having been held prisoner for several years by a Chaos Renegade known as "The Huntsman". She was officially retired from active service, though many years later would be reinstated as a Four Sword Commander.