Tether Essence

Tether Essence is a dunamancy spell created by Matthew Mercer for the Explorer's Guide to Wildemount.

Spell description
Two visible creatures within range must each make a Constitution saving throw, with disadvantage if they are within thirty feet of each other. Either creature can willingly fail the save. If either save succeeds, the spell has no effect. If both saves fail, the creatures are magically linked for the duration, regardless of the distance between them. When damage or healing is dealt to one of them, the same damage or healing is dealt to the other one. If either of the tethered creatures is reduced to 0 hit points or the spell ends on either, the spell ends on both.

Trivia

 * In a Tweet a year before the release of Explorer's Guide to Wildemount, Matt teased that one of the spells dealt with "quantum entanglement", the theory that entangled particles remain connected so that actions performed on one affect the other, even when separated by great distances. The phenomenon so riled Albert Einstein he called it "spooky action at a distance."