Cosmic Car Key

It comes off the Martian Saucer, the gunship that turns up partway through Martian Madness, and that event only becomes available once Golem is down. Nothing else hands you one, so getting it means running the event until a saucer finally coughs it up.

Summon it and you stay airborne as long as you like, with no meter draining in the corner and no landing to recharge. You also keep full use of your items while riding, which is why it is the building mount of choice. Fly to a spot, place blocks, move along, never touch the ground. For laying out an arena or roofing a large structure, nothing else is this relaxed.

It is not quick. Straight lines are sluggish, turns are worse, and there is no dash at all. Diagonal travel is noticeably faster than moving along a single axis, so the habit worth building is holding two directions whenever the route allows it. Momentum also carries into the mount: drop from a real height and summon it while you are already falling fast and you launch off quicker than it could ever accelerate on its own, which is worth doing at the start of any long trip.

• Liquid ends the ride. Touch water, lava or honey and you are dismounted on the spot. If you need endless flight underwater, the fishron mount or the Witch's Broom cover that gap.
• Going up, it squeezes through a single block gap as long as you are also moving sideways.
• Going down, platforms hammered to the lower half of their block stop it dead and it bounces off them. Ordinary platforms let you through after a short pause. This will confuse you the first time it happens over your own arena floor.
• A Rod of Discord covers its worst weakness, since you can blink through tight terrain instead of dismounting.

Because it replaces flight outright, you can strip wings and movement boots out of your slots and put damage or defense there instead. That is the honest argument for it away from building. The counter argument is that hover wings with high tier boots outrun it and give you a dash back, and the Soaring Insignia from the Empress of Light hands you unlimited flight without spending a mount at all. In frantic arena fights you are usually better on foot with wings, or on the fishron mount, which is fast and boosts your damage around liquid.

• Sit still on it for a few seconds and burrowing and teleporting enemies stop spawning around you, which makes it a quiet place to wait out a night.
• You can hand the key to another player while riding and keep flying, so one copy can outfit a whole server.
• Stealth from armor or the Psycho Knife switches off while you are mounted, so do not plan a stealth burst from the saddle.