MBD, MBY, and Seraya
You've heard of Mashiah ben David.
You've heard of Mashiah ben Yosef.
But who's Seraya?
I was first introduced to the concept of Seraya a number of years ago by a Rav in town who mentioned him.
The Zohar (see p. 4641) mentions him as a warrior helper of MBY in his battles against our enemies.
He comes from the tribe of Dan. (Also mentioned in Kol Hator.)
On the verse לישועתך קויתי ה׳, the Midrash says:
לפי שהיה אבינו יעקב רואה אותו וסבור כי שהוא מלך המשיח כיון שראה אותו שמת אמר אף זה מת לישועתך קויתי ה׳
Yaakov Avinu saw him [Shimshon] and thought that he was the King Mashiah. Once he saw that he died, he said, "Even he died? I hope for your salvation, Hashem."
The נזר הקדש doesn't like the גירסא of "וסבור כי שהוא מלך המשיח" here, and instead changes it to match the following paragraph in the Midrash by using the words "וסובר בו שגאולה מגעת בימיו", so Yaakov Avinu didn't think that Shimshon was the Mashiah since Mashiah must come from Yehuda. Rather, he thought that Shimshon would be the mighty warrior from the tribe of Dan that would fight until the time of the Geula. However, Yaakov Avinu then saw that Shimshon would die, and therefore said לישועתך קויתי ה׳ - meaning, "I know that the Geula will come via someone from the tribe of Dan. If not from Shimshon, I hope for your salvation, Hashem, through Seraya, who will come from the tribe of Dan during the time of the Geula Sheleima."
There's also a gematria I found in ויספר משה:
ויפל רכבו אחור בגמיטריא וזהו יהיה שריה