Ether 2:8

and he had sworn in his wrath unto the brother of Jared, that whoso should possess this land of promise, from that time henceforth and forever, should serve him, the true and only God, or they should be swept off when the fulness of his wrath should come upon them.

The Lord made a promise to Jared’s brother when he was angry. He said whoever lives in this promised land must serve him, the one true God. If they don’t, they will be destroyed when God’s full anger comes.

And he made this vow to the brother of Jared: whoever possesses this promised land from now on must serve him, the one true God. If they don’t, they’ll be swept away when his anger reaches its peak.