Half-Life mod "Prison" from Reaktor (creator of "The Trap").
Holy crap, was this a cluster-cuss O_O
The name is a bit misleading, as the prison levels end rather abruptly, leading to very well detailed Xen levels from there on-out.
But these levels immediately turn to frustration, as many of the fights range from incredibly difficult to unfair.
There are, however, new NPC's to fight, such as mini-gargantua's, plus those looking for good Xen level design will be pleased, but be warned, this mod will kick your ***.