Serpent-Coil Braid

Serpent-Coil Braid is an epic trinket for mages.

Source
This item drops from Morogrim Tidewalker in Serpentshrine Cavern. The use of this trinket combined with a Mana Emerald will restore approximately 3000 mana back to the caster.

Effective Damage
The effective +damage from this trinket is difficult to calculate, as it is not always prudent to use Mana Gems the moment they come off cooldown. Assuming one does however, this item adds an additional 225 * (15 / 120) = 28.125 +damage

It is important to note however, that the real benefit of this trinket comes from the additional ~600 mana gained from using a Mana Emerald - which can add quite a substantial amount of DPS for an Arcane Mage who utilises heavy Arcane Blast spam.