Terraria – How to improve the chance of Solar Eclipse happening in Terraria

You can go exploring some corrupted/crimson areas (randomly picked at world generation) to destroy the shadow orbs/crimson heart waiting for you at the bottom of chasms.

Each shadow orb/crimson heart destroyed will provide you some loot, and make a meteorite land on the world. Once the meteorite has fallen, go find the site, and mine the meteorite ore. This will allow you to craft some meteorite gear and weapons.

Once you've destroyed three shadow orbs/crimson heart, it will make the Eater of Worlds/Brain of Cthulhu spawn, allowing you to fight two different bosses.

Note: The shadow orbs are protected behind some ebonstone/crimstone, which you will not be able to mine with your current pickaxe. Just buy some dynamite from the Demolitionist and use it to access the shadow orb. Or you can buy purification powder from the Dryad, which turns the ebonstone/crimstone into the easily destroyable standard stone.

They are considered magic damage by the game. The terraria wiki states that the Terra Blade "is a Sword/Magic Weapon that fires sword-shaped projectiles." Also, if you kill a player with the projectile, it gives a death message that does not contain the word "terra blade" (or similar), rather it gives the name of the projectile, though this is not necessarily related