Mantra of Retribution returns 40% damage to enemies. Serenity makes you invulnerable for 3 seconds. If I am invulnerable from Serenity while Mantra of Retribution is active and an enemy attacks me, will he receive damage from the Mantra?
No, the Mantra will not damage your enemies while Serenity is active.
I just tested this on some low-level monsters, and regardless of which was activated first, the monsters took no damage from the reflected attacks while Serenity was active.