Half-Life 2 just turned 20-years-old, and to celebrate Valve updated the game with some new features. They also produced a documetary in which several of its development team look back on their work on the game and its episodic expansions - including the never-released Episode 3.
The documentary includes in-progress footage of the episode in action for the first time, and it shows an ice gun and a new liquid enemy type.
The discussion of Episode 3 begins at 1h 52m, with concept art and eventually in-game footage. The in-game footage is all taken from test arenas and tech demos, not finished parts of the game.
Various Valve developers say they worked on the episode for around six months, before moving over to the original Left 4 Dead, never to return.
There are two main features of the episode discussed. The ice gun let you create geometry by spraying ice, so you could create impromptu cover to hide behind, which soldiers would shoot through and eventually smash.