Blessed Apple

Unicorns are the only source, which means the Hardmode Hallow and nowhere else. It used to trickle out of most of the Hallow's population at long odds, and now that it comes from a single enemy the chance per kill is far better, so a short farm on hallowed surface ground usually gets there.

Ride it in a straight line and it builds speed for a few seconds until it starts throwing off sparkles. That sparkle is the tell that it has crossed the damage threshold: from then on anything you run into, enemy or critter, takes a heavy summon hit and gets flung a long way, and you pick up a sliver of invincibility on each pass. Below that speed it is only transport.

The charge reads speed you already had, so you do not always have to build it from a standstill. Mounting mid sprint, out of a Shield of Cthulhu dash, off Tabi, or out of a cancelled grapple starts you already charging. Mount during a Shield of Cthulhu dash and connect with something and both the dash hit and the charge land on it.

Anywhere you can lay down a long flat run. Ground events are the obvious case, and with a wide enough platform the Old One's Army stops being a real fight well before you have the gear that event expects. That reach makes it one of the strongest mounts available before Plantera. In a cramped arena you spend most of the fight decelerating and turning around, and wings or a flying mount win instead.

• It ignores Asphalt and a Swiftness Potion, so good boots on a prepared asphalt track actually out-run it on flat ground.
• Every extra jump accessory stacks with the mount's own, and a fully kitted character can chain a startling number of hops before touching down.
• Fall damage is reduced rather than removed, though any wings or a Lucky Horseshoe cover the rest.
• Shallow lava still burns you while mounted, and the unicorn stands taller than most mounts, so low tunnels are more awkward than you expect.
• Dropping after a full gallop is faster than the gallop itself, which is occasionally useful and occasionally alarming.