Military action
Blackwald and Haven having had enough of Phostra's evil ways try to rescue the elves and the dwarves, while Phostra tries to expand.
Blackwald having a lot of dwarven allies and some elves reaches the boiling point politically and starts a war with Phostra, Haven joins in to protect its ally. The war is long and bloody however Phostra comes out victorious and keeps its slave population despite some being freed and escape to Blackwald to live on.