Gloon is an Ancient One designed for teaching Arkham Horror. Limiting investigators to three physical items makes life difficult for the investigators, but it also limits the decision matrix for new players: just pick the three most useful things that you have and run with them. A Doom Track of 10 ensures that the game will be finished in reasonably short order, one way or the other.
Magnum Innomandium makes a simple change to the rules to ensure that there is more combat. As monsters will move every turn, the streets and sky will soon be crawling with all manner of malevolent entities, and woe betide the investigators if they leave any green bordered monsters on the board for too long. This Ancient One is particularly dangerous on boards with vortices, such Dunwich, Innsmouth and, of course, Lovecraft Country itself.
Mordiggan adds a large element of chance into the game. Every player must experience an Other Worlds Phase. Because of this effect, this Ancient One greatly blurs the difference between the traditional Arkham roles such as street sweeper and gate diver.
Tru’Nembra plays with one of the motifs in Lovecraft Country: Music. You’ll need to close (rather than seal) some gates when playing against Tru’Nembra, or you’ll end up (quite literally) facing the music. To use Tru’Nembra you will need to print out about 40 music tokens.