Planet Baalimm, Jorus System
Regular military exercises performed by the elements of Guardians of Celeres 10th Company on their recruitment world Baalimm went horribly wrong. During troop deployment as a part of the drill, novices where supposed to sweep through Gortanis Hive lowest levels and practise infiltration of Hive environment.
Baalimm’s10th Mechanised Inf Regiment was to assist Astartes scouts and deployed few days earlier to the field. Yet as it was revealed to the marines novices, Imperial Guardsmen turned traitor and ambushed the scouts. On the Astartes side the looses were drastic. Not only 58 chapter initiates have fallen in the initial exchange of fire but also accompanying Tactical Squad Singrog was cut down while covering the retreat.
Astartes were outnumbered 30 to 1 and had to withdraw to chapter fortress monastery located on the edges of the Hive. Yet before they got there second in command of the planetary Guardians of Celeres detachment veteran sergeant Tanrus was shot down during organised retreat. Or so did the report say. Much darker truth has been later revealed towards his fate.
During the retreat Marines realised that whole hive went mad. It looked like pre-organised planet-wide insurrection against the rightful rule of the Imperium.