Rantal also served on the Imperium's War Council and was present when the Mechanicum Loyalists' Ambassador Vethorel, presented her plan for the creation of the Adeptus Mechanicus. If the creation of the new Adeptus was agreed to by the War Council, it would make the Mechanicus a part of the Imperium, instead of the uneven partnership it now had with the Mechaniucm due to the fall of Mars. Agreeing to its creation would also end any doubt that the Mechcanicum's nominal Fabricator General Zagreus Kane and not the Heretic Kelbor-Hal, truly led the Omnissiah's servants. Despite these beneficial outcomes however, Rantal and his fellow High Lord Simion Pentasian, both refused the Ambassador's plan. They argued that it would be foolhardy for the Imperium to grant any more power to the Mechanicum, given that the Priesthood of Mars was divided by a civil war that left many questioning its Loyalists' integrity. The other High Lords followed their lead and given they held the most power within the War Council, Vethorel's proposal was denied.
This was not the end however, as once word spread of the Ambassador's intent, a group of Loyalists', led by Magos Passax and Gerantor, attempted to assassinate her. It was only due to the intervention of the Princeps Bassanius of Legio Ignatum and Tevera of Legio Agravides, that Vethorel's life was saved and afterwards the Princeps told her they agreed with her plan for the creation of the Adeptus Mechanicus. With their backing, the Ambassador once again presented her proposal during the War Council's next meeting. However the now irritated High Lords claimed that with the loss of Mars, the treaty between the Imperium and the Mechanicum was void. Further, the attempt on the Ambassador's life, proved the Mechanicum could no longer govern itself and called for the Mechancum's forces to be directly controlled by the High Lords. But as Rental and the other High Lords began to argue amongst themselves, as to who among them should have control of the various Mechanicum forces, the chamber the meeting was being held in began to shake. They did not know it, but Vethorel was no longer going to take no for an answer and had signaled Princep Bassanius to begin advancing his Imperator Magnificum Incendius, upon the building where the War Council was meeting. Now faced with a violent end at the hands of the Magnificum Incendius, and with no Imperium forces close by to stop the Imperator, Rantal and the High Lords were forced to agree with Vethorel's proposal. Before the meeting was ended, the Adeptus Mechanicus and Adeptus Titanicus were formed and Fabricator General Zagreus Kane, was made a High Lord.
During the early stages of the Siege of Terra Harr Rantal was present during a meeting of the Council of Terra within the Great Chamber of the Senatorum Imperialis. He pointed out the rioting and lawlessness affecting Terran cities in the wake of Horus' attack.