Mystic Robe

The shirt on its own is half a package. The Wizard Hat route pushes magic crit, which suits a build that already carries crit accessories and a fast firing weapon. The other hat hands you a flat block of extra mana instead, which matters far more early on when your pool empties in a handful of casts and you are chugging potions. Leg gear is unrestricted with either pairing, so you keep whatever defense you already had down there.

It comes from the Traveling Merchant, so owning it is a question of catching him on the right visit rather than something you can plan around. What you are buying is damage and cheaper casting in exchange for essentially no defense, which is a good trade while enemies still hit softly and a bad one the moment they do not. Drop it as soon as a real armor set comes together.