The Crossing

The Crossing is a secret chapter of Curse of the Blood Elves. Moments after Kael and Vashj killed Kassan, Vashj reopened the portal that Kel'thuzad used to bring the Burning Legion into Azeroth.

Garithos ordered his troops to take explosives from a pile of explosive barrels, ignore the defenses and destroy the portal. He convincingly told them that the one who destroyed the portal would be a hero.

Kael, Vashj and some Blood Elf Engineers needed to hold Garithos forces from destroying the portal to Outland. The Blood Elf Engineers reconstruct the Dalaran's primary defenses, they constructed five types of defensive towers, they started constructing more and more towers with the resources taken from Kassan's prison while Garithos' Elite Guard ran past the towers ignoring them, with the only objective of destroying the portal by detonating explosives at it. After most of Garithos' forces were destroyed all of Kaels forces had passed through the portal, Kael ordered the Engineers to fall back to the portal, then he and Vashj passed through the portal and after that it was closed.

Naga and Satyrs in Outland
In this mission, only Blood elves and Vashj go through the portal, and Illidan passed through a portal on his own — yet still there are Naga in later missions and Satyrs in World of Warcraft. The Satyrs must have followed Illidan after the events of The Frozen Throne, because they aren't seen in Curse of the Blood Elves. Still, there is no explanation of how the Naga went to Outland. They may have somehow passed through a portal to Outland before the Blood Elves.