Moondancer is a magicka-based light armor set. When you activate a synergy, you gain a shadow blessing that increases your Spell Damage by 448 or a lunar blessing that increases your Magicka Recovery by 448 for 30 seconds. You can only have one blessing active at a time.
Moondancer is often used on setups that frontbar one particular set. When you only have this set on the frontbar active, you need to make sure to use synergies on the frontbar, otherwise the buff will not apply. Only one of the two blessings can be active (Spell Damage OR Magicka Recovery but not both), and every time you use a synergy it refreshes the duration randomly of one of the two blessings.
The Moondancer set is part of the Thieves Guild DLC. It drops in the Maw of Lorkhaj Trial in Reaper's March. Each set item is bound on pickup. You can acquire light armor, weapons and jewelry of this set in the Dro-m’Atrha motif style.