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My players obtained a Robe of the Archmagi, which grants them this
You have advantage on Saving Throws against spell and other magical effects.
What qualifies as a magical effect? Is it only effects of spells?
What about those of magical items?
Arrow of Slaying: The creature must make a DC 17 Constitution saving throw, taking an extra 6d10 piercing damage on a failed save, or half as much extra damage on a successful one.
Or abilities of creatures, like Dragons?
Frightful Presence: Each creature must succeed on a DC 21 Wisdom saving throw or become Frightened for 1 minute.
Fire Breath: Each creature in that area must make a DC 24 Dexterity saving throw, taking 91 (26d6) fire damage on a failed save, or half as much damage on a successful one.
Or effects of potions?
Potion of Poison: If you drink it, you take 3d6 poison damage, and you must succeed on a DC 13 Constitution saving throw or be Poisoned.