Mana Devourer

Dalaran
Climb out of the sewers and clear the city's terrace, gardens and commerce district. Find Arcane Anomaly on the terrace and Unstable Sentinel in Magus Commerce Exchange; include Arcanic Enigma for Power Overwhelming. Clear the enemies around Mana Devourer before pulling it, and collect the city quest objectives before going up to the Violet Citadel.
Mana Devourer
Drain Vitality pressures the tank while repeated Debilitate casts consume the group's mana.
Abilities

Drain Vitality
Continue healing the tank after the initial hit because Drain Vitality also deals Arcane damage over time.

Debilitate
Hits an enemy with a bolt of draining arcane power that siphons up to 200 mana.
Begin with mana restored and commit damage cooldowns early so repeated mana siphons do not exhaust the healer.
Tank- Clear Mana Devourer's surrounding enemies before the pull. Use mitigation for Drain Vitality to reduce the healing needed while Debilitate removes the healer's mana.
Healer- Enter with mana restored and reserve enough healing for Drain Vitality's ongoing damage. Avoid spending the mana budget as though Debilitate were only a health hit.
Damage- Start this fight with resources ready and use damage cooldowns before repeated Debilitate casts drain the group's mana. Clear the surrounding enemies before committing to Mana Devourer.