Valve has started trailing Half-Life 2 expansion Aftermath ahead of its September release. The video is more of a teaser really, and hints at darker themes (and, as if to illustrate this, is also really quite dark and at points hard to make out).
We can see "stalkers", which seem to be rather unpleasant, and Alyx features heavily - including a shot of her putting down an Antlion, presumably an unfriendly one. It's an act that reminds us we haven't mentioned the Antlion Troopers Deuce mod yet. Now we have. Go Antlions!
Half-Life 2: Aftermath, of course, deals with the what-happens-after of City 17's Gordon Freeman-instigated downfall, picking up as the city starts to crumble. It's part of a larger canon of post-HL2 work that Valve's undertaking, which also includes Source Engine remakes of popular Half-Life 1 modifications like Day of Defeat and The Lost Coast, which reshapes a coastal section of Half-Life 2 using (more) advanced graphical techniques like High Dynamic Range (HDR).