I understand this sentence contains: 'they will bring for themselves quiet/still/rest' and 'ἀγαθῇ μοίρᾳ' is clearly 'good fate' in the singular dative. What I do not understand is the role of the dative. It would not make much sense for it to be the indirect object or a dative of respect, so I would think of a dative of means or more probably of manners, so it would be 'They will bring for themselves quiet in the manner of good fate', which would mean something like 'It is their fate that they will achieve peace of mind', but I am not sure. What do you think?