Mothron

There is no summoning item. It arrives on its own during a Solar Eclipse, but only after Plantera is dead, so every eclipse earlier in Hardmode is a wasted trip if this is what you came for. Only one can be alive at a time, and the Lifeform Analyzer will call it out as soon as it spawns.

It ignores terrain completely and flies through blocks, so no arena you build will keep it out. The pattern is to chase, then line itself up on your horizontal level and ram. The speed of that ram varies and so does the damage it does: a slow drift into you is survivable, a full-speed charge from across the screen usually is not.

Flat open ground is the right call, since you want room to move sideways and time to see the wind-up. Wings cover the vertical dodges, or a Gelatinous Pillion if that is what you have. An alternative that works better than it should is dropping down a hellevator and shooting straight up while falling, because it cannot line up a horizontal charge on a target that keeps descending.

Every so often it lands and stops dead to lay a cluster of eggs. Those few seconds are the entire fight. It is completely motionless and takes everything you throw at it, so this is where your damage belongs and where you should be holding your strongest cooldowns.

Break the eggs before they hatch. Baby Mothrons drop nothing, crowd the arena, and add chip damage while you are trying to dodge charges. One trap to avoid: if it lays on a single block floating in mid air, it can sometimes vanish without dropping anything at all, so keep isolated platforms out of your arena.

It is also easier to hit while charging, since it flies into your projectiles instead of away from them. Weapons that punish that, like the Crystal Serpent with its splash, get far more out of the fight than their damage line suggests.

Ranged and magic are the comfortable answer because staying at distance is most of the fight. The Piranha Gun is the fastest option in the game here and ends it in well under a minute. Venus Magnum, Chain Gun, Tsunami and Snowman Cannon all work, and on the magic side Razorblade Typhoon, Blizzard Staff and Nightglow.

Melee is harder because closing the gap is how you die. The Death Sickle is the fix if a previous eclipse already gave you one, since it keeps damaging through the egg-laying window and holds the thing at bay for a few seconds. Vampire Knives trade that for lifesteal. A venom or ichor flask is worth applying either way.

For armor, take defense over damage. Ranged and magic players do fine in Shroomite, Chlorophyte or Hallowed. Melee players want Turtle or Beetle, because absorbing one bad charge is the difference between a kill and a corpse run.

If the eclipse ends or every player dies, it flies straight up and disappears. It also becomes invincible on the way out, which no other enemy does. Nothing you do in that window counts, so if the sun is about to come back, either finish it now or accept the loss and wait for the next eclipse.

Its headline drops carry hidden requirements. The wings, the eye and the broken sword all check that the mechanical bosses are down, and two of them check for Plantera on top. In normal play that is automatic, since you cannot meet it any other way.

Despite hitting like a boss it is not one. No boss health bar, a banner rather than a trophy, and no relic in Master. The name bends Mothra, the kaiju from the 1961 film, into the suffix Terraria hangs on so many of its monsters.