@JeremyECrawford Vampire Regeneration vs. Spirit Guardians? What is the order of operations for "at start of turn" spells/abilities such as Vampire's Regeneration starting his/her turn inside a Spirit Guardian's of an enemy?
— Matthew (@Mattnetic) November 16, 2017
Xanathar's Guide contains a rule on simultaneous effects. Short form: you decide the order of simultaneous effects on the turn of a character/monster you play. #DnD https://t.co/X9gUe7KXER
— Jeremy Crawford (@JeremyECrawford) November 16, 2017
Is you the spell caster or the dm here? "You" is the person controlling the character/monster whose turn it is.
— Jeremy Crawford (@JeremyECrawford) November 16, 2017
Was this really answered?
yes. the character suffering the effects decides in what order they happen