Heartreach Potion

Hearts normally have to be walked over, which means healing pulls you out of your dodge pattern at exactly the wrong moment. With this running they come to you from most of a screen away. In a fight where you are circling an arena, the hearts you left behind on the last lap get swept up on the next one without you steering for them.

The big invasion nights are the obvious case. Things die faster than you can collect what they leave, hearts expire on the ground, and a large share of the healing on the field is simply lost. This turns that waste back into sustain, and during those events it is frequently worth more than a flat defense buff. It is also strong in any farm where you stand in one place and kill things at range.

One of the ingredients is a fish that only lives in crimson water, so a corruption world cannot brew these as it stands. The workaround is to buy crimson seeds from the Dryad once hardmode and a graveyard let her stock them, grow a small patch somewhere it cannot cause trouble, and fish there.