All of New Cerberex's population and infrastructure was now firmly under Tau control, but the continued Dark Angels presence at Mount Bone rankled the Tau high command. Although the Dark Angels were effectively contained within their mountain fortress they represented a point of vulnerability should the Imperium ever try to regain the world. The fire cast commanders also wished to report their total victory to the Ethereal council. A previous attempt to capture the mountain had ended it catastrophic casualties for little gain so the Tau proceeded with caution.

They began by probing the outer defences with small reconnaissance cadres, seeking week points in the defences but the mountain was surrounded with a web of sensor nets and one after another the scouting forces were ambushed by ravenwing or devastated by teleporting terminator attacks from the deathwing who were guided to their positions with pin point accuracy.

The Tau did identify one location where the defences might be breached and committed a linebreaker cadre to exploiting the weakness. Just such a tactic had worked well in New Feducia and the Tau hoped that success could be replicated here. As the heavy battlesuits closed on the mountain they were met with an aggressive counter attack. The attack stalled in its tracks as the battlesuits engaged, wary about closing the distance and determined to exploit their advantage in fire power. Fighting raged and casualties mounted on both sides but eventually the battlesuits were forced to withdraw. Both sides had sustained heavy losses but the aggressive response had kept the Tau from pressing forward to claim the key positions they would require to breach the fortress.

Disheartened the Tau commanders analysed the data that had been gathered with much sacrifice. The fortress itself was a subterranean stronghold, immune to even orbital battleship bombardment, and its key position and surrounding sensor web made any approach impossible to conceal. Taking the mountain would require prolonged attractional warfare of a kind that was anathema to the Tau philosophy of war. All analysis lead to the same conclusion, that the military benefit of taking the mountain was simply not worth the cost. Faced with this unpalatable conclusion the Tau moved to a strategy of containment and refocused their resources to future conquests.