Could not use instants, Timing issues
I was playing against a slivers deck, with no permanent or abilities preventing me from playing instants. I could not play Sejiri shelter or Flare of Fortitude as a reaction to cascade or attack by opponent's creatures. Only when the opponent finished all actions did the game allow me to play instants. I had enough mana up for Shelter and I had non-token creatures for Flare.
When I activated Flare in opponent's next turn's main phase, and I didn't take any damage, the ability of a sliver that says that they get +1 counters after doing damage still activated
forestlander#97213
shared this idea
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Rezzahan#77802
commented
Can't say anything about the first problem without a screenshot.
The second issue, however, was handled correctly by Arena. Flare makes it so your life total can't change, it does NOT prevent damage. You did take damage, the damage just didn't change your life total due to Flare. And since damage was dealt, the trigger went off.