Wormhole Potion

Multiplayer only, and specifically for staying together. Drink one, open the full map, click a teammate, and you land beside them. It only works within a team, so set teams at the start of a session or the potion does nothing for you.

Boss fights you cannot walk away from. The Wall of Flesh is the clearest case: die once and you respawn on the wrong side of the world with the fight drifting further away every second. A wormhole puts you back in the fight immediately. The same logic covers anything fought in a fixed arena where the walk back costs more than the death did.

It is also the cheapest travel network available before Pylons. Leave one player parked somewhere useful and everyone else can hop to them on demand.

Slimes drop them as a bonus, but only when the world is running in multiplayer. Nobody else actually has to join: hosting a session alone is enough to switch the drop on, which is how most people build a stack. The minimap has to be enabled or the map click that triggers it is unavailable, and where a Pylon and a player sit on top of each other, the Pylon takes the click.