A. In an Elder Thing’s Area, Cthulhu can’t Devour. He can still use Submerge and Y’Ha Nthlei, as those are Spellbooks, not Great Old One special abilities. This ability also works on Independent Great Old Ones (but only their special abilities, and not their Spellbooks). Nyarlathotep can’t use Harbinger, Azathoth can be Killed with a single Kill result, and so on. Shub-Niggurath can’t Avatar out of an Area containing an Elder Thing, but she could use Avatar to enter an Area containing an Elder Thing because she is not in that Area at the moment she uses her Avatar ability. (And the player controlling Bokrug could lose his Loyalty Card!)
In some cases, Mind Control can cancel the abilities of Great Old Ones. For example—if Tsathoggua is in an Elder Thing’s Area, then he can’t use Lethargy. If Ithaqua is in an Elder Thing’s Area, then Ferox is not in effect for any of his Cultists (of course, if an Elder Thing is merely in the same Area as one of Windwalker’s Cultists, but NOT in Ithaqua’s Area, Ferox is still in effect). Likewise, Abhoth could put a Filth Token in an Elder Thing’s Area, so long as Abhoth himself does not share an Area with an Elder Thing. Some Spellbooks might be affected, even though they are not directly negated by Elder Things. For example: The King in Yellow can’t Desecrate, which means that his Third Eye Spellbook can’t be used, either (a similar effect happens to Cthugha and Ghatanothoa when they have their respective Firestorm and Execration of Mu Spellbooks).