After all, from Onyxia's perspective, she had merely been following orders. Back then, Neltharion, the Aspect of the Black, had wanted to bring down all the human kingdoms from within, and he had partly succeeded.
After the Second War, Prestor, a great noble of Alterac, had successfully driven a wedge between Dalaran and Stormwind City, Lordaeron, Stromgarde, and Kul Tiras. At the time, Neltharion had also kept feeding Terenas and the others the idea that Dalaran posed a threat.
But Neltharion had still underestimated Terenas's political skill. What did Terenas value most about Prestor? The fact that Prestor had lost all his lands and had no army whatsoever. The Alliance of Lordaeron had just settled accounts with Perenolde, and the shattered Kingdom of Alterac needed a king to hold it together. Terenas also needed a puppet to control Alterac for him. Lord Prestor, who had only a single servant at the time, had become Terenas's favorite candidate for that puppet.
Whether Neltharion had factored this in was unknown. Perhaps his claim that only one of his servants had survived had merely been intended to conceal his past and keep others from investigating his background. It was also possible that Neltharion had deliberately arranged things this way. After all, the Aspect of the Black had long occupied a position of power. To those in power, a puppet who could only rely on others was always easier to trust than a highly independent noble. The Barov Family was the clearest example, and Patrick himself leaned more toward the latter interpretation.
Lord Prestor had neither an army nor lands. To ascend the throne, he needed someone backing him, and the powerful Lordaeron of that time had been the best choice. Even if Prestor turned against Terenas after his coronation, he had neither troops