Bolas understood that overwhelming force was not enough. He needed a way to gather many planeswalkers in one place, keep them there and convert their sparks into power. Each element came from an earlier conflict. Tezzeret controlled the Planar Bridge. The Immortal Sun had been recovered from Ixalan. Amonkhet's Eternal army waited beyond the reach of ordinary travel.
On Ravnica, political pressure did the preparatory work. Agents and allies interfered with guild leadership, encouraged mistrust and made coordinated resistance harder. The city was not conquered before the invasion began, but it had been arranged so that every disagreement would consume precious time.
The plan reveals Bolas at his most dangerous. He was not relying on one unbeatable artifact. He combined people, institutions and magical tools so that each part covered another's weakness. To defeat him, the defenders would have to dismantle that design while trapped inside it.