Adurdin's 1st Levelpack
Adurdin's 1st Levelpack features only one custom Keen 2 level, which is set as level 29 (taking over the Washington Tantalus Centers spot on the map). In the beginning of the level, there is a sign that says "Mothership Control Room Authorized Persons Only". The worldmap was edited to change how some of the levels look, including ones that don't lead to a custom level when entered.
Spoiler warning! This shows the whole level.