The Ishmaelites were rogue Shemsu-Heru that abandoned the crusade of Horus for a variety of reasons. They are named after Ishmael, the first Shemsu-Heru who spoke against Horus' leadership and who was banished to walk eternity alone. Most Ishmaelites left Egypt and traveled across the world and when the Diaspora was called, many Shemsu-Heru found out that their banished cousins had various advantages in the foreign lands they had wandered through.
Ishmaelites are – after all- still Shemsu-Heru, the only difference between them is their allegiance. The Ishmaelites see Horus as a petty tyrant whose small-mindedness blinds him to greater evils of the world. They are loyal to Ma'at in most cases and many were opposed to Apophis' schemes, but they simply saw the way Horus had chosen as ineffective. After their death, Ishmaelites were not allowed within Amenti, except to weigh their heart by the Judges of Ma’at.
After the Sixth Maelstrom, many Ishmaelites were destroyed and became tem-akh. Some however, survived the wrecking of the Underworld and continue to exist as Shuankhsen.