The Cross of Mercy is a melee weapon added by Xeno's Reliquary. It functions similarly to a sword, with the exception that upon striking an undead creature (skeletons, wither skeletons, zombies, the Wither), it unleashes a "holy blast of concussive force" that damages and knocks back all undead around the target.
It can be found in top-tier Battle Tower chests.
Recipe
the Cross of mercy