Thalyssra is in Shal'aran and if you don't have portal on top [spell=202664] entrance is near the flight master. Look for 2 pillars.
Coordinates of the cave entrance are ~34, 47
Don't forget to take [item=136269]. It is near Falanaar 21.0, 54.5 + 100 [currency=1155] manapool
Subject 16 is part of Nightfallen But Not Forgotten - Arcanist Kel'danath storyline.
You can find more info about storyline in the achievement link
You will also need this quest for Good Suramaritan
Here is Subject 16 video: https://www.youtube.com/watch?v=REE5Azz5KRQ
And here is storyline related to this quest:
7
https://www.youtube.com/watch?v=VuoAYJhRpmU
15
https://www.youtube.com/watch?v=FNg9HJqIs0g
16
[item=140349]
https://www.youtube.com/watch?v=pjxhV4a5MA8
17
[item=140349]
https://www.youtube.com/watch?v=19fjPfcRXVw
18
https://www.youtube.com/watch?v=RrO993DJURk
19
[item=141668]
https://www.youtube.com/watch?v=REE5Azz5KRQ
20
https://www.youtube.com/watch?v=Ac2Ys22Pk5s
№ column is quest approximate order in Suramar storyline
btw there is Shal'an rare and it is <Subject 12>