what is happening here
Oh, the days when Dwayne Johnson still was credited as "The Rock".
As for whats happening, there was a prophecy that whoever kills the Scorpion King gets control over his army of sand warriors (which he got by selling his soul to Anubis). Immotep wants the army to take over the world, Brandon Frasier is just pissed Immotep killed his wife (who was revived, because magic) and wants to kill Immotep in return and was in the right place at the right time.
Youre seeing Brandon Frasier spear the 2002-cgi Dwayne Johnson and Immotep's reaction.
Is that the mummy?
The Mummy Returns.