Glimpse of the Temporal Flood vs Orros Mark of Fate

Glimpse of Temporal Flood from the Precognitive Mage says 

Glimpse the Temporal Flood . The mage targets one creature within 120 feet of it that it can see. The target takes 18  psychic damage, and it must succeed on a DC 14 Intelligence saving throw or be stunned until the end of its next turn.

Stunned prevents a creature from taking reactions.  

However Orros Mark of Fate states 

Which has priority?   

Mage casts Flood - Sorcerer fails, does the stunned take effect before they can react or does it activate Mark of Fate first?  

Thanks in advance. 

The Reaction takes effect immediately when the saving throw is failed, turning it into a success before any of the effects of failure are applied. That's the whole point.

There's basically two stages to checking if a roll passes or fails- the first one involves applying modifiers that are applied "when you make an attack/skill check/save", then the result of the roll as it stands is given and there's a window to use reactions with triggers like "on a fail" or "on a hit/miss".

In terms of RAI, it's likely as described in the above responses.

In terms of RAW, it gets a little weird.  The general rule for a Reaction is that it is "an instant response to a trigger" but "in terms of timing, the Reaction is taken immediately after the trigger".

So when a feature says "it must succeed on a saving throw or be stunned", it doesn't seem like reacting "immediately after" or "in response to" this is possible.  At best we might be looking at Simultaneous Effects which also would not work out in favor of the one making the Saving Throw in most cases.

If you could actually get to the text which reads that "something else happens instead" then it's as if the original consequences never happened, but RAW I'm not sure how you get there in this case.

The general idea we went with is,  for the Mark of Fate to trigger, the save must be failed, which applies the stunned condition, preventing the use of a reaction.