Flower of Fire

The ball leaves the wand slowly and keeps ricocheting off surfaces until it expires or finds a target. It does not pierce, so one ball is one enemy, but in a corridor those bounces let a single cast sweep the tunnel several times over. Out in the open most of that is wasted and the low travel speed lets anything mobile walk around it. Aim at floors and ceilings rather than at the enemy.

This comes out of Shadow Chests, which puts it in your hands right after Skeletron and well ahead of most of its tier. Against the Wall of Flesh and the busier parts of the Underworld it holds up fine, and the burn it applies about half the time matters more than the hit itself against low-defense swarms. Its ceiling is low, though, and the Flower of Frost is the Hardmode version of the same idea.

You cannot fire it at all while submerged in water or honey, and any ball that touches either dies instantly. That rules it out for the Ocean, flooded caves and the lower Jungle pools. Lava is fine in current versions, which was not always true.

It is the Fire Flower from Mario, bouncing fireballs and all, and it carried that name outright for the first few versions. The Wizard throws the same projectile at anything that wanders near his house.