"Warchief, I am willing to join the Horde." After meeting the Soulbinder Blackhorn, the black dragon prince Wrathion ultimately returned to Orgrimmar and chose to join the Horde.
He regarded the Soulbinder Blackhorn as his last remaining family, while Blackhorn unhesitatingly chose to continue serving the Tauren tribe he had been loyal to for countless years. Now that the Highmountain Tauren had joined the Horde, Blackhorn naturally became a member of the Horde as well. With his last family in the Horde, joining them was no longer so hard to accept.
In truth, Alexstrasza and the others had been too naive. They believed that purifying the black dragon prince Wrathion would allow them to purify the entire black dragon race. That was impossible.
Only the descendants of Wrathion and the Soulbinder Blackhorn would be unaffected by the Old Gods' curse.
But... Wrathion and Blackhorn were both male. How could the two of them breed black dragons unaffected by the whispers of the Old Gods? Since no uncorrupted female black dragons existed, they could be said to be the last surviving descendants of the pure black dragons.
However, Blackhorn told Wrathion that there was a way to purify those black dragons corrupted by the Old Gods: rely on the power of another Old God to absorb the tainted energy.
None of the three surviving Old Gods—the Thousand-Mawed Demon Yogg-Saron, the Thousand-Eyed Demon C'Thun, and N'Zoth—would do such a thing. Their only hope lay with Xal'atath.
In other words, so long as Blackhorn and Wrathion demonstrated enough value to the Horde, they could ask Rend to have Xal'atath help purify a female black dragon.
The possibility of restoring the Black Dragonflight gave Wrathion hope. Before this, he had chosen a carefree, wandering life because what use was there in being the sole