Suspicious Looking Eye

Summoning is the only way to take the first real boss on your own schedule, and that matters more than it sounds. Waiting for the natural spawn means the fight starts while you are halfway down a cave with a chunk of your health missing. Craft one, get your arena and your potions in order, and start it when you are ready.

If the game warns you that something evil is watching, the fight is already queued to happen on its own that night. Using the item at that point does not cancel the queued spawn, it adds to it, so a second one arrives shortly after the first. That is a fast way to double your loot if your gear is ready, and a fast way to die if it is not.

You do not always have to make one. Cavern chests turn them up often enough that a thorough early dig usually hands you at least one before you have gathered the materials.