Suggestion is a 2nd-level spell that charms creatures for 8 hours and RAW does not give them any subsequent saving throws. Can it be used to turn someone that is otherwise on good terms against one or more of their allies?
The Rise of Tiamat has the following use of suggestion:
The malisons also use suggestion spells to tell characters that a fellow party member has been replaced by a yuan-ti spy and should be attacked. (The Rise of Tiamat, p. 46)
If you suggest for instance, "Your friend the Warlock has been replaced by a Doppleganger that is going to destroy you. Kill it now, before it has the chance!" Would the character on a failed save be forced to fight their ally, thinking they have been replaced by a Doppleganger (or yuan-ti spy)?
For comparison, Crown of Madness is a 2nd-level spell that charms creatures for 1 minute, must spend your action maintaining each turn, and gives saving throws to resist each turn. It seems unreasonable that Suggestion could be used as a more powerful Crown of Madness, but the idea in The Rise of Tiamat seems to suggest it would be possible.