This is "Residual Point", dev. from 2004 to 2007 by Korean team Radiation studios, led by Toxagen.
This acts as an alternate storyline to Half-Life, you play as Jack Sohn, a Ph D MIT Theoretical Physics Decided Major, Research Assistant working at the Biodome Complex. He gets involved with teleportal experiments and then as usual all hell breaks loose. It is unclear whether it's because of you or Freeman in Anomalous Materials that Resonance Cascade occurs, as A.M. are mentioned by some characters.
This chapter list below uses the 2020 addon/update and the dev. commentary translated from Korean and helped by what I saw during my playthrough :
1) Intro : tram ride, the dev tried to make it less "boring" than usual as some people sometimes feel it this way, this was already the case in 1998, you know, even in computer magazines (I remember...).
2) INSECURITY : Black Mesa, the dev. talks about exploring "useless" areas, there is indeed an uneventful and worthless office. they created and are proud of an elevator that can move without a loading screen.
3) INCOMING : For the dev, this place has "unnecessary but curious areas", indeed the Biodome is a far more pleasant place to wander than Anomalous Materials, with big water tanks filled with leeches or (for now...) placid Ichtyosaurs. This is the Resonance Cascade, our role is less direct than in White Force (pushing the crystal cart), here we are the guinea pig in the experiment.
4) AMONG THE RUINS : "Unforeseen Consequences" rip-off, the dev acknowledge that we can quickly get lost here in this labyrinth of collapsed parts, and regrets it.
5) OFFICE FACILITY : a sometimes disliked but essential part of HL, the "office complex" is here very close to the original.
6) OBSCURE TRUE : HECU's arrival, "We've got hostiles" . The complex staircase in the center foreshadows their love of deeply interconnected levels. It was inspired by Blue Shift.
7) RESIDUE PIT : « Blast Pit » ideas mixed with "Residue Processing" setting, created for the purpose of having at least one long puzzle (involving pumps and stuff). One lift at the end has the property of rewinding.
8) BOMBING SURFACE : mix between "surface tension" and "Forgot about Freeman", air strikes destroying parts such as bridges. It is left to the player to fight or not the ospreys, I choose sometimes to do it, sometimes not. There are sand fields full of headcrabs... foreshadowing "Chaotic Butcher" of Residual Life. This was supposed to be a flooded part, with Ichtyosaurs.
Two alternatives paths :
9a) RECESSION "PARKING ROUTE" : Dev. tells how he managed to make bigger maps by using change_target instead of changelevel. This map is full of black ops, they are the "left overs" after the military evacuation/escape. About the transparent barnacle, it was to show why people suffer inside... You have to escort a scientist and it is annoying to save and protect his 1998 A.I. and *ss .
9b) RECESSION "SEWER ROUTE" : Rip-off of HL2's canal. Flooded zone with Ichtyosaurs everywhere and you don't have enough ammo, so it's more trial and error... a nightmare for those who were and are still terrified by this alien Jaws lookalike. Some complex path where you have to use O2 systems underwater otherwise well you die... The dev has nothing to say about the complains that there are too many Ichtyo.
Fun stuff like here with a large battle around Biodome complex with Assassins jumping in water and fighting Ichtyosaurs... this is not scripted and I think it's just a random event unexpected by the devs.
10) DISTORSION : an interesting twist about the tram ride where you have to fight on the way... There is no proper comment or explanation about Gman's appearance...
11) UPPER YARD : for the dev, a very long (maybe too much) chapter in terms of scale and number of maps. This is just fights after fights in regular Black Mesa, harder than ever.
12) COURSE TO LAMBDA : it was planned to be a straight course with not too many enemies as we are so close to go to Xen.
13) INTO THE PORTAL : The dev says this is the only time a map from HL1 is directly used in this mod. Xen's maps were created through "trial and error", and as in Residual Life, we have very large maps but not so many oppositions, in fact we walk and run for a long time but there are a few battles. However, it's one of the most polished work
14) DREAD PLOT : Nothing interesting in the dev comm, but there is here interesting things, it's a rip-off of Interloper but it's not a Alien grunt factory, it's here you witness how Zgrunts are made, and you also see them in RL... There is also a very fun tram ride but with alien stuff and alien trains and tracks. The only and final boss fight sadly comes out of nowhere and puts you against 2 Gonarchs, it's not that hard compared to some previous battles...
The red teleporter was the original ending, but now this is where you can get access to the 2 last levels, by destroying the 2 purple cells on the back of the teleport.
15) RESIDUAL POINT : the eponym chapter of the mod was originally a cut level... you are back on Earth but the sky is Xenish and/or portal storms, Manta Rays flying freely in the sky... you face Black Ops, HECU, Zgrunts.
16) FINAL ESCAPE : The very last chapter is surprisingly short and features an escape by riding a serviceable military truck... Originally seen by the dev to be "an unnecessary waste of time", finishing on Xen was seen as "not too bad", and so it has been deleted (available through console... but the very end was broken until today...). The ending will bring some closure to those who played it 12 or 13 years ago and wondered if this was truly worth it in the end.
To be continued and ended in Residual Life.
