Each abhorrent can use his summoning ability once.
Majestic Horror means that when the character with it uses his command ability it doesn't cost a command point. It used to do something quite different last edition.
The Charnel throne means that any foot Abhorrent (AAR or Ghoul King on Foot) uses their command ability it doesn't cost a command point.
Any units you summon have to be within 6 of the board edge and more than 9 from enemy models.
So your GKoTG could summon in either 3 flayers or 3 horrors. If he has majestic Horror it doesn't cost a command point, otherwise it costs a command point.
The AAR can summon in any of the courtiers, 3 horrors/flayers or 20 ghouls. If he is on the throne it doesn't cost a command point, if he has majestic horror it doesn't cost a command point. If neither of those things is true then it costs a command point.
That is all they can summon that game.
