The Powers may only be appeased, and the curse lifted, by a willing sacrifice (to be precise, one willing sacrifice for each Power).
Hathil's idea is to trick Gair (who, through a misunderstanding, he thinks is Orban's son) and Gerald into volunteering. This probably wouldn't have worked, but fortunately is not in the end necessary.
Hafny, horrified by his father's plan, hands himself over to the Lymen, volunteering to be killed in retaliation for Gair's death. Gest and Adara won't hear of it, but Hafny's willingness appeases the first of the Powers.
When it seems that Hathil will be obliged to take the curse upon himself, Gair, feeling sorry for him, genuinely volunteers his own life if it will lift the curse. This appeases the second of the Powers.
Hathil, what with one thing and another, declares that he will not do anything to Gair - or to Gerald - even if it does mean the curse rebounds on him, and even though it means giving up his revenge. This appeases the third of the Powers, and the curse is lifted.
Adara's vow bears fruit, as her wisdom and learning are part of what guided them to this point.
Hathil's vow to spill Orban's blood is part of what he gave up as his sacrifice to appease the third power; and his additional condition, that the Powers should not be satisfied while any of Orban's people remain alive on the Moor, is subsequently fulfilled when Orban and Kasta, unable to stomach the new spirit of co-operation between Lyman and Dorig, take their family and leave the Moor forever.