2 Chronicles 15:9
And he gathered all Judah and Benjamin, and the strangers with them out of Ephraim and Manasseh, and out of Simeon: for they fell to him out of Israel in abundance, when they saw that the LORD his God was with him. And he gathered all Judah and Benjamin, and the strangers with them out of Ephraim and Manasseh, and out of Simeon: for they fell to him out of Israel in abundance, when they saw that the LORD his God was with him.
The strangers who stood with Judah and Benjamin were people from Ephraim and Manasseh and Simeon. They came over to Asa in abundance, joining Judah and Benjamin in large numbers. They did so because they saw that the Lord his God was with him.
He gathered them together with all Judah and Benjamin at Jerusalem. The gathering was part of his reformation, accompanying the people's assembling in Jerusalem. There they entered into covenant to seek God wholeheartedly and to renew their commitment to Him.