Spirit Flame

Casting drops a flame somewhere in a small ring around you and it simply hangs there. It only moves once an enemy comes near and has clear line of sight to it, then it chases and bursts with a splash. Nothing stops you casting several in a row, so the pattern is to build a cloud of waiting flames before anything arrives and let them all commit at once.

The spawn point is random, and if the game rolls a spot inside solid rock too many times in a row the flame appears in the wall and dies instantly. Your mana goes either way. In a tight tunnel that happens often. The flames also need to see their target, so anything behind a block is ignored until you both round the corner. Give it an arena and it is close to free damage; use it in a corridor and you will burn a lot of mana on nothing.

The damage is out of line with how early you can have it, since none of it is gated behind a mechanical boss. What it does need is a Corrupt or Crimson Underground Desert, which your world may not have generated as an overlap, plus Sand Elementals that only turn up during Hardmode Sandstorms. That is a lot of waiting on weather in a biome trying hard to kill you in early Hardmode gear. If the pieces line up it carries you a long way, and it is cheap enough to reforge that a good modifier is easy.