The lost ten tribes will produce the third witness that Jesus is the Christ. Their record will contain the story of Christ’s visit to them after his ascension and visit to the Nephites, But now I go unto the Father, and also to show myself unto the lost tribes of Israel, for they are not lost unto the father, for he knoweth wither he hath taken them (3 Nephi 17:4).
This branch of Israel is by far the largest, at least by the number of involved tribes. They were led north after the Assyrian captivity and have since been referred to as the Lost Ten Tribes. It is apparent from the scriptures that many of these lost Israelites were scattered all over the earth. It is equally apparent that a main body retained its identity and heritage. Proof of this is seen in the fact that they were a cohesive body that the Lord could visit in the meridian of time (3 Nephi 17:4), that they had their own prophets who would record this visit and other prophecies (v. 12), and that they will return as a cohesive unit from the north to receive their temple blessings at the hands of the Ephraimites (DC 133:26-32). Their brethren who have been scattered are meanwhile being gathered by the great missionary effort of the latter days.
Bruce R. McConkie
"’We have no knowledge of the location or condition of that part if the Ten Tribes who went into the north country.’ (Compendium, p. 88.)
"Esdras, an apocryphal writer, records this version of their escape from Assyria: ’Those are the ten tribes, which were carried away prisoners out of their own land in the time of Osea the king, whom Salmanasar the king of Assyria led away captive, and he carried them over the waters, and so came they into another land. But they took this counsel among themselves, that they would leave the multitude of the heathen, and go forth into a further country, where never mankind dwelt, That they might there keep their statutes, which they never kept in their own land. And they entered into Euphrates by the narrow passage of the river. For the most High then shewed signs for them, and held still the flood, till they were passed over. For through that country there was a great way to go, namely, of a year and a half: and the same region is called Arsareth. Then dwelt they there until the latter times; and now when they shall begin to come, The Highest shall stay the stream again, that they may go through.’ (Apocrypha, 2 Esdras 13:40-47)
"…The Lost Tribes are not lost unto the Lord. In their northward journeyings they were led by prophets and inspired leaders. They had their Moses and their Lehi, were guided by the spirit of revelation, kept the law of Moses, and carried with them the statutes and judgments which the Lord had given them in age past. They were still a distinct people many hundreds of years later, for the resurrected Lord visited and ministered among them following his ministry on this continent among the Nephites. (3 Ne. 16:1-4; 17:4.) Obviously he taught them in the same way and gave them the same truths which he gave his followers in Jerusalem and on the American continent; and obviously they recorded his teachings, thus creating volumes of scripture comparable to the Bible and Book of Mormon. (2 Ne. 29:12-14.)
“In due course the Lost Tribes of Israel will return and come to the children of Ephraim to receive their blessings. This great gathering will take place under the direction of the President of The Church of Jesus Christ of Latter-day Saints, for he holds the keys of ’the gathering of Israel from the four parts of the earth, and the leading of the ten tribes from the land of the north.’ (D. & C. 110:11.) Keys are the right of presidency the power to direct; and by this power the Lost Tribes will return, with ‘their prophets’ and their scriptures to ’be crowned with glory, even in Zion, by the hands of the servants of the Lord, even the children of Ephraim.’ (D. & C. 133:26-35.)” (Mormon Doctrine, pp. 455-8)