It's tricky from the perspective of the individual lords; the line between "region changing sides" and "realm changing sides" gets hard in very small realms.
At the same time, this seems pretty cut and dry. This is clearly a collaboration between Alaster and Pierre to merge the whole realm in in a friendly fashion.
Total eradication is friendly?
This isn't like when Luria Nova absorbed the other Lurian realms, to basically continue the Lurian Empire thing they had going, leaving everyone with their prestige and titles. D'Hara is not friendly to Terran. D'Hara doesn't want to preserve or honor Terran. And Alaster doesn't get to keep anything out of it. Honestly, I'm not even sure he'd be allowed to stay in D'Hara even if he tried. He's basically sabotaging his own realm, alone. It also isn't as if the realm is viable: he's not chosing to destroy what could have survived as an independant realm, he's simply chosing the conqueror he prefers. Remember:
Realm mergers are illegal. Realms may surrender to another, including annihilation of their lands, but they may not merge as equal entities on friendly terms.
This is, in my eyes, a clear surrender, not a merger. They do not merge as even remotely equal entities. Alaster did try to get a bunch of concessions from D'Hara at first, but was plainly told that would not happen. And the Kabrinski family are hardly friends to D'Hara.
