Half-Life-Lambda Core

After much struggle, Gordon finally reaches the Lambda Complex, which he discovers to be the place where Black Mesa developed the teleportation technology that allowed scientists to travel to Xen in the first place.
The handful of surviving personnel, who have held themselves up in a small stronghold, inform Gordon that, the satellite he launched earlier was not able to reverse the effects of the Resonance Cascade because an immensely powerful being on the other side is keeping the rift between the worlds open, and Gordon must go to Xen to kill it and prevent the aliens from taking over completely.
Before being teleported to Xen, Gordon gains access to an arsenal, health and HEV-charge caches, and he is finally reunited with the Long Jump Module. After an attempted interception by Alien Controllers, the scientists open a portal and Gordon jumps in.