Undaunted Infiltrator is a medium armor set. When you use an ability that costs Magicka, your Light Attacks deal an additional 774 damage and Heavy Attacks deal an additional 1161 damage for 10 seconds.
Note: The heave attack bonus applies to every tick of heavy attacks, that makes the set especially powerful with Lightning Staves, because of them channeling heavy attacks.
The Undaunted Infiltrator set is part of the base game. It drops in Arx Corinium in Shadowfen. Each set item is bound on pickup. You can acquire medium armor, weapons and jewelry of this set in the Breton motif style.