That'd be merfolk, as they urged Mr Arterxaxes, I mean Ertaxarxes, oh, whatever (that is not Rover only to have problems with spelling of his name, you see

) to come out and 'do something' about Sea-Serpent
On the other hand, that may be Sea-Serpent itself, as it tried to make Mr Ataxerxexexs stop doing abovementioned 'something', that is, mighty spell of his, causing insomnia in Serpent
edit: Cross-poster here, Squatter

. You have 7 minutes in your favour, so take it away as Esty confirms it (that is, I do believe it is pretty safe bet we've got correct answers)
BTW, what does 'autothalassic' stand for?