Rainbow Rod

Hold the button and the missile trails your cursor wherever you drag it. Let go and it launches along whatever direction you were dragging, or straight away from you if the cursor was sitting still. From there it homes on its own, punches through a couple of enemies, and detonates on the next one or on the first wall it touches. While you are still steering, walls do nothing to it, and neither does water.

You pay once at the cast. Steering costs nothing, and your mana regenerates the entire time you are guiding the shot, so the real firing rate is set by how long you spend flying each missile rather than by the size of your pool. Keep one alive while you reposition and it is effectively free.

The missile lights up the ground around it and passes through terrain while under control, so it doubles as a probe for scouting caves without moving yourself. The catch is your inventory: opening it, or just sweeping the cursor over a slot while it is already open, drops control and flings the shot off in whatever direction you were last moving.

Homing plus a blast on impact suits big slow targets, so Skeletron Prime and Plantera are the natural fights for it. Against small fast enemies the steering overhead costs more than it returns.