Black Mesa: Definitive Edition - 15a - Xen [Hard]

Chapter 15: Xen - Part One

After jumping into the Lambda Reactor's portal, Freeman arrives on Xen, the alien borderworld where an immensely powerful being known as the Nihilanth is keeping the rift between the two dimensions open. To confront Nihilanth and put a stop to the alien invasion, Freeman must navigate the vast voids, dense jungles and flowing rivers of the borderworld to make it to the ominous tower looming in the distance. Along the way, he discovers the remnants of the previous expeditions by Black Mesa, whose crews have all perished to the hostile Xen flora and fauna.

Playlist with all completed chapters:
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Notes:
Jumping onto the floating rocks directly in front of Gordon at 1:36 allows access to a secret pathway to skip the starting Xen island, placing him at the bridge at 3:07. Doing so also unlocks the achievement "Orbital Trajectory"
Protozoans (2:07) make their return from HL1 but now approach Gordon if they are close enough. They are not hostile but can impede player movement. Killing all of them in the Xen chapters awards players with the "Prolific Popper" achievement
The alcove at 8:22 is where the Pizza and Purple Hat is teleported to, if players brought them through the Lambda portal for their respective achievements
The HEV Zombie (11:36) is a new addition to BM. It has more health than a regular zombie thanks to the suit but can still be easily dispatched with a headshot. The HEV Suit voice can be heard desperately trying to keep the user alive and combat the Headcrab's control
If Freeman brings the cyanogen canister at 14:25 with him all the way to Gonarch's Lair, he can kill it using the gas following the instructions on the board at 14:11, which also unlocks several hidden achievements and a secret passageway in the final stage of that chapter. However, I didn't want to carry a canister through the entirety of Xen (wouldn't be much fun for you guys anyway) so I opted out of this method
The Xen Trees in BM can be killed (19:57), as opposed to their HL1 counterparts
Two new Houndeye variants are introduced in BM: Exploding Houndeyes at 23:52 and Armoured ones at 38:57. Both did not exist in HL1

Trivia:
Black Mesa's Xen is radically different than HL1's variant. Whereas BM's Xen comprises of 6 maps and places significant emphasis on the various ecosystems in place on the borderworld, HL1's Xen only comprises 1 map (the teleporter island that is featured as the 6th map in BM's Xen) and can be completed in a much shorter timeframe
Though Gordon grabs the crowbar at 0:12, the game will automatically switch his weapon to the last active weapon used before jumping into the portal in the last chapter
Various Xen flora from HL1 makes their return, such as the Light Stalk at 0:46, Fungus at 7:27 and Xen Tree at 18:30
Nihilanth's Tower (0:50) is not present in HL1. It's addition to BM is reminiscent of Valve placing the Citadel in view during the City 17 Uprising in HL2 to show Gordon's progression through the story
If Gordon takes the secret path at the starting Xen island quick enough, he can see that the Manta at 1:03 clips into a rock and disappears
Much like HL1, dead HEV Suit survey members are scattered throughout Xen and Gonarch's Lair (1:54). BM adds a distress beacon to their suits to allow players to more easily locate them. As well, many of the survey crew have dev team names printed on the suits
The healing pool heals Gordon much faster than in HL1 (4:43)
The scientist at 5:04 ironically dies right next to a Healing pool
Blue Xen crystals are a new addition to BM. This suggests that Black Mesa also obtained their HEV charger fluid from Xen (8:01)
Killing a HEV Zombie by shooting off its Headcrab reveals that the top of the helmet is cracked and the brain is exposed (11:51)
As shown by the whiteboard at 13:17, Xen Crystals are used to power local teleportation devices. The only known exception was sample GG-3883, which caused the Resonance Cascade
The whiteboard at 14:11 details the survey team's plan to kill the Gonarch, encountered by Freeman one chapter later. Freeman can actually follow this plan through by bringing the cyanogen canister on the table at 14:25 to the teleporter at Gonarch's Lair. Killing the Gonarch this way nets the achievement "The Plan"
The giant Tree at the map starting at 28:47 can be seen in the distance one map earlier at 23:41

The whiteboard at 12:33 contains a ton of information on what happened to the survey members, as the names on the boards correspond to the tags on many of the dead HEV scientists found in this chapter:
Team 1 perished on the first map of Xen
Team 2 ventured into Gonarch's Lair to kill her, only to perish (they are listed as MIA). Team 3 was sent in to recover them but also all perished as seen in the next chapter
Team 4 perished in the swamp