The warlord/sorcerer Menthu depicted here shares the name of an Egyptian god who was a god of warfare, sun, and valor.

Queen Isis, who was previously Mathayus' ally in the battle against Memnon in The Scorpion King, is now an adversary, teamed with Menthu.

Menthu and Isis seek the Scorpion Stone, which grants great power to those who possess it. The Scorpion Stone appears to be a fictional relic.

Mathayus seeks out the Hero's Gauntlet, which can aid him in destroying the Scorpion Stone. The Hero’s Gauntlet appears to be a fictional relic.

Mathayus also recovers the Sword of Osiris, the only known weapon which can kill Menthu. It's not explained what happened to the Sword in the time since Rise of the Akkadian or how it came to be found where it was located here. Mathayus is depicted as having to quest for it again, so apparently he did not keep a hold of it after the events of Rise of the Akkadian.

In the Dunes of Nath, Mathayus confronts Menthu and Isis. As far as I am aware, the Dunes of Nath are a fictional region.

After killing Menthu, Mathayus faces Queen Isis, who uses the Scorpion Stone to transform herself into a half-human, half-scorpion creature to battle him. She essentially looks like the creature Mathayus himself will one day become as seen in The Mummy Returns.

As noted in the synopsis of The Sword of Osiris at The Mummy Chronology, there are two alternate histories within the game besides the main one. In the main history, Mathayus' queen, Cassandra, is kidnapped by Menthu and Isis, so Mathayus sets out to rescue her. In the first alternate history, the player is allowed to play as Cassandra, and depicts Mathayus kidnapped by the evil pair instead while Cassandra completes the quest to free him. The second alternate history occurs if the player fails to defeat Queen Isis at the end of the game and she flees with the Scorpion Stone; in this scenario, Mathayus and Cassandra vow to continue their quest to destroy it.