Creeper Egg

Light pets are the good kind of pet. You keep both hands free, you never burn a torch, and the Flickerwick lights the ground around you as well as itself, so it reads more like a permanent glow on your character than a lantern trailing behind. It also matches your speed exactly, so it does not fall behind on a long run the way some pets do.

Ogres drop it during the Old One's Army, and your odds are better in the second tier than in the third. There is no reason to grind the harder waves if this is the only thing you are after.