Amulet of the Night

The mount turns you into a small bat with flight that never runs out. In a normal world that is a Hardmode convenience. In a Vampirism world it is the earliest unlimited flight anyone gets, and by a wide margin. It is slow, though. Walking beats it in a straight line, so think of it as a way to cross broken terrain rather than a way to outrun anything.

The form is fragile in a very specific way. Swinging a weapon, placing a block, touching water or taking fire damage all drop you straight back into your own body, so you cannot fight from it and you cannot cross an ocean with it. Getting caught in rain is the one that surprises people. The bat hitbox is also tiny, which means you can squeeze into gaps you cannot dismount inside. The game retries a couple of times, then teleports you to the nearest space that fits, or back to spawn if there is nothing close by.

Vampires drop it during a Solar Eclipse, but that seed also tucks copies into ordinary chests, so you can be airborne within the first hour. Spare copies sell for a lot, which is the fastest early money the seed offers. Sunlight does nothing to you in bat form, there or anywhere else.