Relic
Mage's Serenity Banner
Unique
Effect
After Stall (15 and Stall 30), 
Mages gain 100 Mana.
Where it drops
No drop source recorded in the game files.
Other Unique relics
Questions people ask
What does Mage's Serenity Banner do in Guildrun?
After Stall (15 and Stall 30), Mages gain 100 Mana.
What is the difference between a relic and an item?
Items are equipped by a single hero and take up one of that hero's slots. Relics apply to the entire guild and do not use slots. That makes relics the stronger of the two when a build starts coming together.
How do I increase my team size?
Harmony Crystal raises it by one, up to five. So does each of the fifteen Mandate relics, but those require finishing an objective first, like dealing 15000 auto attack damage or applying 30000 shields. Pick the Mandate that matches what your team already does.