The Cursed Child is found inside the Revered Beacon dungeon, which you can find inside the Abbey in Mortal Shell 2. The dungeon involves a gimmick where you need to avoid the rotating light from atop a tower in the middle area. Make your way up to the top of this tower, all while avoiding the light and beating back enemies, and the Cursed Child is yours to claim. Unlike other Sidearms in Mortal Shell 2, this one acts more like a lantern than a gun, and you can do some pretty interesting stuff with it once you get the right Tarstones.
