Firstly, the background for the adventure is very well written, and seeps out character and flavor. It was plainly well thought out and very developed. It is excellently layered with a hook that does not belie the true nature of the adventure, and a twist that should keep player interest throughout the endeavor. Good setup and thorough execution of initial hook. Also, there is good characterization of various expository NPC's that promises a memorable session if a GM is willing to act them out.
From the first encounter, it seems like encounter design and balance are well done for the sensibility of 5th edition. The initial combat is meant more as a test of the character's sensibility than their combat prowess, and could lead to some interesting decisions later. Statistic blocks are handled rather nicely, and are concise and readable. The next few encounters are a bit standard, and probably a sync for a 2nd level party to deal with, but it really builds up the atmosphere, especially with the special glyphs that seem to permeate the enemy lair. Room and encounter design do a nice job of setting the tone of the adventure by slowly introducing a creepy undertone.
There are interesting little details peppered within the room descriptions of the enemy lair, including some interesting moral choices regarding enemies that do not resist or fight.
The inclusion of a new creature (the negative energy elemental) is refreshing, and a bright contrast against the relatively mundane hobgoblin foes, as well as foreshadowing the nature of the lair itself.
The methodology for controlling the number of creatures in a later encounter was interesting as well; depending on player actions, they could fight more enemies at once, or fight a few with more enemies entering per round. This seems to reward players for circumstantial actions rather than punishing them for making an unknowable mistake.
Ultimately, the adventure is very short, and could serve as an excellent set piece to be fit into a larger campaign. There are a lot of loose ends presented within here that could either be tied to your campaign, or simply expand the adventure to interesting directions. This adventure will most likely take your 2nd level group to 3rd level (depending on how many there are in your party), so that makes it ideal for moving your characters into an ideal starting point for a full on campaign (given that the accepted wisdom states that adventurers are more sturdy and defines at level 3).
All in all, this excellent and short adventure is a showcase of an excellent understanding of 5th edition design and low level balance! 5 stars and my royal seal!